Why was the Nostromo the only one?

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.