Initially the idea seemed to be Libressl has better development practices => fewer bugs. The idea was this pays for a compatibility price. I think that didn't pan out very well longer term.
https://marc.info/?l=openbsd-misc&m=168674593912952&w=2
and the reply:
https://marc.info/?l=openbsd-misc&m=168675478419177&w=2
Edit: certainly, if you don't need what isn't there, LibreSSL could be a viable replacement.