Because it takes 60 votes to vote on anything in the Senate and the Party of No won't allow it, legislation to correct the Supreme Court's damaging ruling on corporate political spending will fail. (The cloture vote drew a healthy 57 votes, but majority rule is meaningless in the Senate now.)
It's real simple. Republicans want unlimited amounts of corporate money spent on elections and they want utter secrecy about the source of that money, even if it is laundered foreign cash.
It's OK to require disclosure of the little guys, the chumps whose names are disclosed in campaign spending reports. But the really big bucks? Move along. Nothing for you peons to see.
Will the Fox Noise chamber convince the angry white men that total secrecy about unlimited special interest spending on elections is a good thing? Answer: Probably. They're lemmings.