Why was the Nostromo the only one?

Basically because there was a agenda.Perhaps theres not much cause for an Android on a mining vessel or should i say "commercial towing vehicle".

Honestly, there's great reasons to keep an Android on any ship. It's an insurance policy to have an intelligent being capable of handling minor repairs, maintenance or course corrections that might become necessary over the course of the flight. 

The Nostromo was a commercial towing vehicle, but it was still a very expensive ship.

What happened to make the risk outweigh that benefit? Did an entire model line begin acting in ways that made Androids on board a liability, or at least distasteful to the human crews?

It's clear that by the time the Nostromo departs, people are very uncomfortable with androids. There is a distinct distrust. Perhaps whatever happens in and post Covenant, leads Humanity to turn against their androids and view them as troublesome beings rather than helpful aids.

I love the theory that Weyland-Yutani are actually ruled by Androids at its highest point, which would explain the disregard for Human life and extreme fascination with a seemingly half organic, half mechanical organism. Some people have grown wise to it and thus oppose Androids being a part of the crew. Possible?

I like it.

Androids on board ships were standard procedure in the Aliens timeframe; not Alien.  Especially not on a tug.

Why was the android a secret on the Nostromo? Good question.

It's hard enough to answer when treating Alien as a sole film, yet even more so when considering the other films.

The way I see it, I think we should treat Ash like a spy; someone who isn't really who he says he is, and infiltrates a target to monitor/manipulate/extract information for the benefit of his masters.

The company knows about there being 'something of interest' around the vicinity of LV-223--for argument's sake, let's say they sent specialist teams back after Prometheus but found nothing--so now they just insert spies in normal commercial operations, just in case they stumble upon what they've always been looking for (which they do of course).

@S.M. They put a synthetic on a tug anyway. They just wanted the fact hidden from the crew.

@Stan Winston that makes sense. I just always got the impression that they hot swapped Ash at Thedus because they KNEW about diverting the Nostromo. 

My question is why did Androids STOP being standard procedure?

"Why was the android a secret on the Nostromo? Good question."

They wanted a specimen of whatever was being warned about on LV-426, and were willing to put the human crew in jeopardy to get it.

"@S.M. They put a synthetic on a tug anyway. They just wanted the fact hidden from the crew."

They put a synthetic on the crew for a singular specific purpose.  Considering that purpose, of course you'd want it hidden from the crew.

" I just always got the impression that they hot swapped Ash at Thedus because they KNEW about diverting the Nostromo."

No need to have an impression - the film makes that very obvious.

"My question is why did Androids STOP being standard procedure?"

We don't know what standard procedure is circa 2122.  What standard for an exploration or colonisation mission, might not be for a tug.

The company placed Ash there to ensure the capture/collection of the specimen. An android can be programmed to do whatever it's creator wants, therefore eliminating the "human" aspect of the situation (Crew expendable....any human would have a moral dilemma, not a synthetic)

I know why Ash was there. I'm wondering why in Prometheus, and apparently Covenant, synthetics are standard operating procedure, but on the Nostromo no synthetic was expected to be on board. 

It may have been a tug, but it was still a $42,000,000 ship. That justifies a synthetic to manage things while the crew sleeps. So why not assign one instead of sneak one in?

Possibly because Prometheus and Covenant were both scientific missions and the Nostromo was basically just "space truckers" on a tug?

And they had no need for a synthetic to manage things while the crew was asleep. They had MU/TH/UR. And she worked out ok :-P

Lol

"It may have been a tug, but it was still a $42,000,000 ship. That justifies a synthetic to manage things while the crew sleeps."

Clearly not.

Yes, but it seems like this is fertile ground in context of the other movies. One of these things is not like the other. What is different about it? Keep in mind that "tug" is carrying and refining lots of oil. the reason for lack of androids is not monetary.

Bishop wasn't awake on the way to LV-426 (though he was originally scripted to). 

Could also be because of the events in these films (Prometheus and Covenant). Humans came to distrust synthetics because of David/Walter and what they came to stand for. So putting a synthetic on board without the crew knowing could make their "job" easier.

According to the marketing behind Prometheus David units got more and more 'social' as they went along.  With the Prometheus mission being classified, there's no way anyone would know he was a little flaky (at least until we know what happens in Covenant).

Just as Holloway had a disdain for David right off the bat, even with David playing nice to begin with, humanity may not like/care that the units are more social. People as a whole dislike different.

Holloway was also a bit of a dick, and not necessarily representative of humanity as a whole.  Shaw didn't have any in built prejudice towards David.

True lol.

But he represented that distrust of something that's manufactured and possibly untrustworthy.

And it was also a reoccurring theme of Prometheus that creation comes to creator only to discover they aren't wanted.

i think this is just one of those things overlooked as alien was the original film they didnt have to tie into anything at the time(i.e android being standard procedure etc) then after alien they became standard procedure because of what happened but in prequels i think they overlooked tying this fact into alien..itcould be the only real reason david was abroad the prometheus was because weyland needed david to communicate with and do his biddings whilst he was in cryo.

i dont believe at this period it was standard procedure

Could be any number of reasons.

Great conversation guys, thank you.

@S.M. EXACTLY. That's what makes it fun!

My thoughts exactly Xenowarrior1.Ash was implemented by David Giler and Walter Hill in there rewrites,and was largely ment for shock value.At the time i don't think anyone had an idea it would become a franchise.That being said,i would like to see some reasons why they didn't have one in the first place.

according to the weyland files which seem to be regarded as canon, the company knows of the signal on lv426, weyland notes that's lv426 appears to have greater rewards but at higher risk. this would say to me they have always been intent to go there and the nostromo is maybe the first ship they control that is planned to go anywhere near that system that wouldn't arise suspicion from rival companies or authorities as theyd have to register flight plans etc. this allows them, due to clauses in the contract, to redirect the vessel. they would obv want someone/ something on board who is on theyr side to ensure they get something worthwhile. and they would def want someone they can control which would def make an android the obvious choice. ash is hidden so as not to raise suspicion among the crew as it seems they don't usual have one

And it fascinates that they don't. Weyland files also say putting David there was standard procedure, but when the Nostromo went out it wasn't. Just curious to see if they'll fill in that blank, or how the community does.

its most likely down to mistrust due to something happening. possibly due to the events in the alien franchise. vessels sent out to explore and no one returns, rumours begin to circulate about the androids role in theyr demise due to the existence of the engineer and xeno is never revealed to the public 

That's a very interesting take on it.  I like it.