Because it's been going on forever?
Pubbies get busted, Dems get to point fingers. Dems get busted, Pubbies get to point fingers. A few players get tossed to the wolves, the others breathe a sigh of relief that they didn't get caught. And We The People keep voting them back into office. So it goes.
Pubbies get busted, they are forced to resign in disgrace or are voted out and replaced by primary voters; Dems get busted, they circle the wagons and cry racism or have their buddies in the media make excuses for them, and their primary voters send them back to office because for dems corruption is a badge of honor.