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.
Considering the fact that the OpenBSD guys are complaining about the OpenSSL guys competance in their commits I don't think they WANT them near the new fork.
