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 mob

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.