We also went from:
Internet Explorer, because it was preinstalled on Windows, and Windows was the most abundant platform to access the internet,
to:
Google Chrome, because it's preinstalled on Android, with Android being the most abundant platform to access the internet now

Chrome also rode on the wave of being "the hip new thing", being a new browser alternative to the previous options of IE, Firefox, Safari, Opera, etc.

But invariably I'd chalk up part of the bulk of it being purely just a shift to mobile devices.

Also, using Firefox or Chrome was reactionary to IE being poorly maintained, whereas you had so much more development capabilities in other browsers, as well as them being far more performant and customizable. There's not as much of a shove that'll push the average person to into install Firefox to replace Chrome on an Android phone, for example, as compared to using ANYTHING to replace IE.