BioStar was a Stack Exchange 1.0 site. Before Stack Exchange operated the current network of sites it allowed others to license the software and run their own sites. BioStar is not part of the Stack Exchange network and now runs their own Q&A software.
See this discussion on BioStar about the possibility of them joining the Stack Exchange network, although at this point I'd be surprised (it's been a long time since an SE1.0 site has joined).
That notwithstanding, you can still use your Stack Exchange OpenID login by manually entering its OpenID URL on the BioStar login screen:
edit: At least in theory... I'm getting an error when I try.