#2 No point in doing it immediately

What's the point of Seth Rollins' heel turn right now?
No, seriously, WWE literally have a month and a half before the start to the road to WrestleMania and there's no need to do anything big right now. Now, that doesn't mean fans wouldn't appreciate a big move being made right now, but what would be the overall point?
Again, it would be somewhat of a shock to finalize the heel turn that everyone believes to be in the books. But that will end a pretty interesting storyline prematurely. Beyond that, it wouldn't give WWE a chance to keep the viewers invested in seeing what happens next.
It's no secret that WWE viewership takes a bit of a dip in the winter months for various reasons and Rollins' slow heel turn might be WWE's remedy for it. At least this way, WWE can keep the speculation going rampant, build up the payoff even more and eventually make a move that will have a monumental impact on the company.
If nothing else, there is no point in finalizing the heel turn right now that could cost the overall interest of the fans.