Why is Opera not free software?

A few years ago, Opera stopped charging money for its browser, but they would show you ads at the top of the screen while you were using it. If you wanted to get rid of the ads, you could pay them.

Then they removed the ads and made the ad-free Opera available gratis.

My question is – why is Opera not free software? Why can’t I download the source code to Opera and modify it, study it and distribute it?

Often I see people bemoaning the lack of Opera support for things on forums, blogs and such.

Releasing Opera as free software would get it into the hands of a lot of people, and Mozilla Foundation seems to be making a ton of money doing just this, so why not Opera?