Because there is an inherent mismatch in privacy concerns and law enforcement / defense concerns.
Office of Censorship was created to counter the German / Japanese threat of spies during World War 2. It was seen as a necessary measure to help unite the country. The successful use of the Atomic Bomb is credited to the Office of Censorship, who helped keep the project secret. Quite possibly... the Atomic Bomb's details would have been leaked in WW2 if it weren't for the Office of Censorship.
COINTELPRO was then created in the 50s to counter the Soviet threat. In the 70s it was revealed it was being used for more than just that, and the project dismantled. In fact, FISA protections were supposed to prevent any large-scale domestic spying. The entire point of FISA was to stop future projects as they rose up.
As PRISM came up, despite FISA protections. Its important to realize how it happened. If you've been paying attention to Congress, FISA has in fact worked: Senator Wyden and Senator Udall have been criticizing these programs for a very long time. No one seemed to care in 2007 however, so the programs continued. I guess Edward Snowden has made things far more dramatic, and easier for the Press to talk about... but these are facts that have been discussed for some time now.
If there is a fault in FISA, its because Americans don't like listening to Senators. So no matter how many Senators you put in charge of watching the Intelligence Agencies, apparently no one will care unless a giant controversy is brought up by a non-politician.
Every few decades, peacetime makes us forget that the world is willing to attack us. Eventually, we as the citizenry work to dismantle the annoying and invasive defensive measures we give to law enforcement. (see Clinton Era in the 90s, where he reduced the size of Intelligence Agencies by half)
Then, an attack happens. The US returns to "war mode", and everyone is willing to trade privacy for security... at least for a few years. And that is when these new programs find their way into the system once again. The creation of Patriot Act led to the authorization of PRISM.
The important issue to see here, is that these programs are 100% legal. These agencies are always willing to follow the letter of the law. So... all we have to do is petition Congress to change the law. Section 215 of the Patriot Act: "Business Records Provision" is the law that legalizes the collection of Metadata. If it dies, the program dies with it.
