Quote:
Originally Posted by
Furienna 
I agree with those, who has said it was because Peter was considered weaker than the other marauders. Well, that was true actually. But they assumed that because of his weakness, Voldemort would never want to use him, that he would have preferred Sirius or Remus as his spy.
This is a really good reason. And also, in my favorite chapters in that book, Remus was never told about how Sirius made them change secret keepers.