Beyond the legal issues mentioned on a sibling post, the main thing in my opinion is that anything requiring installers was killed by Apple transforming the mobile market. People came to expect that most services would be web-based, or uncontroversial enough to be in the Apple AppStore. That expectation destroyed entire categories of software: sure, you could still eke out a living on the desktop, kinda; but any chance at real popularity was wiped out. This is particularly true for non-professional software, because households became mobile-first.

Chilling effects of the "nothing to hide" era. People were being allowed to be sued based on IP, so a lot of people just stopped using the net for normal human things.

The revelations of domestic spying made it obvious how much collusion and voluntary cooperation was going on between the military, federal and local governments, law enforcement, and corporations, and how much people feared the state of unequal information dominance.

Remember, sharing mp3s helps the terrorists win.

Retroshare is still being updated and used (there's at least a few hundred on right now). It's relatively feature complete, and you can route it over i2p or tor as well (which is uncommon for this kind of tooling, in my experience).