Because he doesn't think its his own division anymore?
Why did so many great fighters (including SRR, SRL, Anderson Silva, Mayweather, Pacquiao) change weight divisions? Answer those and you'll know why GSP is playing around with other weight divisions.
Beyond that, its pretty common for athletes, even stars, to play around with where and what they play. Babe Ruth started out as a very successful pitcher before becoming an outfielder. Jordan quit basketball for baseball for a year. Its mainly fans who think players should never try anything new. And then if they don't try anything new, ironically, other fans complain that they're scared to change.
So here's the situation for GSP:
1) Some fans say he's scared if he doesn't fight at 170.
2) Other fans say he's scared if he fights at 170.
No matter what he does he'll be called a coward by people who've never had the guts to try to fight in the UFC, so I suspect he doesn't give fan reaction much thought.