Because directly receiving money is already illegal. Anything currently legal is so because it's protected by the same things protecting local organizations putting up flyers/billboards/radio ads/etc. Even stricter monetary limits don't really work, as you end up catching things such national humane society ads, because if they contain any messaging regarding support for legislation/wanting new legislation it's considered lobbying.
It's really just an intro to the subject, but Knowing Better has a great video on it. Great leaping off point. The very short and very inadequate TL;DW is essentially that "get the money out of politics" doesn't actually mean anything.