Yeah if there indeed is some kind of theoretical reason that makes NP-complete problems hard to parallelize, that would be interesting to know about and understand. Maybe somebody has studied this problem.

I think that unlike the story with neural nets, there wasn't some faction trying to say that parallelizing SAT is a bad idea. In fact the state-of-the-art sequential solvers use techniques to exploit properties of CPU caches to get their speedup breakthrough; that in itself was an important lesson within the research community to pay close attention to hardware capabilities. My impression is that ever since that success within their discipline, SAT specialists became interested not just in algorithms/formal theory but also hardware and implementation, and in today's case that would include having a close look at GPUs/TPUs for the possibility of new algorithms.