I haven't seen an explanation for it, but one reasonable guess would be momentum. If you want to get work done, you don't want to have to wade through red-tape and bureaucracy to get it done. You don't want to work on something with the risk of it being shot-down by the current maintainers. This is not a criticism of the OpenSSL team, it's just a fact of life that it's often easier to "start fresh" than to try to change an already established routine.

I also suspect that part of the reason is because OpenBSD just doesn't work well with others.