But there are some technical limitations to porting the entire framework to OS X. Not the least of which is that a lot of the .NET Framework is a wrapper for Win32, which doesn't exist on any Mac platform (either PowerPC or Intel). As far as I know, the specific database clients use OLEDB to connect to the databases, OS X only has ODBC.
But major parts of it aren't, hence the ported core. And you could have the database objects step down to ODBC transparently for example.