Bombardier was the low bidder, and probably had to promise to not only build the cars in the US but to build them in the Bay Area. So there's something for non-techies to work on for a while.

If they were the only bidder, that could be because there are 1) so few transit equipment companies operating in the US that they can afford to be picky; and 2) they were the only ones (because of previous work) to be familiar enough with the system's needs to be willing to deal with them.

I still think BART should have been Brunel gauge. Or maybe Pennsylvania (transit) standard gauge (5'2.5"). But what we got is what we got and it ain't gonna change. You need some different windmills to tilt at.