Cause in book 1 when Hagrid takes harry to dumbledore on privet drive he flys in on a bike and says
Young Sirius Black lent it me
so at this point i'm presuming black hasn't tried to confront petigrew
Also if no-one except black and petigrew knew that it was petigrew that was the potters secret keeper and not black then at this point dumbledore and hagrid would have thought that it was sirius that told voldermort where the potters lived.
so why would hagrid borrow the bike off of him?