The difference being, of course, that Bobby was a free agent, Murray is one year into an expensive three(?) year deal. I suspect too that his wage demands would exceed BZ's (no evidence for this, just a gut feeling), and of course B'muff would want a substantial portion of their £4 million quid...
A loan in January may well have been a good quick fix, but it didn't happen for whatever reason, so we move on.
Signing him now he would presumably be wanting a two, or even a three, year contract, and I don't see that as building for the future.