My own two Sinators, Kyl and McCain, have been quite passive on this subject, even though much of it took place in their back yard.
There are probably a number of factors, but I suspect many politicians get sucked into a deadly pattern: they become aware of some minor wrong-doing by some other politicians, but are convinced to look the other way. They then become aware of some more serious wrong-doing, but the crooked politician reminds them that if they expose him, they'll also expose their earlier failure to do so. So, rather than risk such embarrassment, they cover for the more serious wrong-doing. The pattern continues until it's clear that the crooks belong in jail, but by that point those who covered for them also belong in jail (and thus still want things to stay covered up).