I think he didn't want Floyd earlier, due to it being a bad move from a business standpoint. Floyd obviously stands a really good chance of making him look bad. He's made tons of other fighters look horrible.
If Hatton had fought him right after Tszyu and lost badly, it would have taken HUGE momentum out of his career. Also Floyd brings a hell of a lot more money to the table today than he did just a couple of years ago. Now Hatton has enough wins to carry on a lucrative career even IF Floyd thoroughly defeated him (I'm not saying that's a given, just very possible any time a fighter gets in the ring with Floyd). Also the risk/reward is alot better now.