Acrylic vs Polycarb - actually, that's an interesting question (another would be why bother having pax windows at all ... ).
Like you suggest, I'd guess cost is a big factor. Acrylic is strong enough to do the job, so why spend more on polycarb. I think some military jets use polycarb.
As for fire safety, I've read that the breakdown products of acrylic are merely water and carbon dioxide, so maybe that's a factor too. I haven't done the home experiment ....
As far as mechanical properties go, the real question, I think, is how do acrylic and polycarb compare in repetitive loading [I don't know], and how do they fail.