"Why is so much ire aimed directly and uniquely at Citizens United, out of the entire body of campaign finance law?"

Asks Howard Wasserman at PrawfsBlawg (via Instapundit):

Some of it is that the Court had to overturn precedent, but... this was not a particularly venerable precedent and it was only one in a broader body of case law. Some of it is the process — relisting, ordering new briefing, etc.... Some--and I suspect a lot — of it is recency bias — the most recent case is the most important case and the one to praise or criticize....

Another theory: Criticism of that case merged nicely with the pervasive liberal rhetoric scaring us about the power and influence of corporations.