"Ash is a goddamn robot!"

I assume its basically what u say. commercial mining vessels were probably all human crews as it may have been too expensive for that particular company to have a synthetic on all their ships plus theres no real need for them. human labour would be cheaper to mine and transport the ore. remember also by the time aliens takes place over half a century has gone by and she probably only ever remembers synthetics being on high end company vessels who could afford them and not onboard military vessels

It appears for a lot of missions a Synthetic was the Norm...

But the back ground for Prometheus does indicate that Problems Arose with Behavior of Synthetics so that People had Concerns about the AI.

Walter Models came out as a Improvement to the Potentially Sentient Flaws in the David Models with more Restrictions.

Ash was sent in as a Secret Operative, and the Reason for this is maybe because the COMPANY can Program a Android with a Mission and they would CARRY IT OUT to Completion.   A Human Operative has the potential to be Swayed against this Mission if they realize the Jeopardy/Risk to the Crew.

Another thing is the Prequels show us the Company would have been some what aware of the Organism, and so that a Synthetic would be able to Handle the Organism with less Risk than a Human which the Organism would use as a HOST.