OpenBSD has been fairly niche for most of it's existence. They got SMP support in 2004, but that was 8 years after the project started. They just recently upgraded much of the network stack to use multiple processors more efficiently (or at all?) when processing packets, and that that's one of the core use cases for OpenBSD. In a perfect world, they would have done all this long ago. I'm pretty sure it's just a matter of not enough developers (or not enough developers of enough skill) to accomplish all the things they want to do.