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.