It's basically because they're so locked down, hard to get docs, stupid toolchains and ides like others have mentioned.

It's like fpga companies don't want people using them, much like others like the pixart sensor I wanted to use: NDA because some parasite dipshit executive or manager thinks that register layouts are extremely sensitive information.

I've had dozens of uses for an fpga...but every single time I just can't be bothered. Why, when they make it a pain in the ass on purpose.

None of these things are true for the new, cheap Chinese contenders.