Compatibility. You want to have enough users to be considered for being paid off, but not too little that you can be ignored. It's similar to the linux gaming move: on linux you can't just add a windows compatible interface to the kernel, so you have to translate it. Game developers thus focus on windows and ignore linux since there build process is completely different. As a browser, you sure as hell can introduce a common interface --> extension devs write their extension once and it run on firefox too. Users who care enough can thus switch without much hassle.

It's a numbers game.