What makes it hard to monetize browser extensions? [closed]

Question

With the monetization of mobile apps being so popular, I'm surprised that extension developers are still mostly relying on donations as their primary form of compensation for their software.

While this is, of course, not a problem, I find myself wondering why browser extensions and plugins are generally exempt from monetization unlike plugins designed for software like Visual Studio and Photoshop are very often available only with purchase.

What makes broswer extensions different, and has anyone had any success charging for a browser extension?

Relevant: http://www.quora.com/Monetization/How-do-browser-extensions-monetize