A person had to use IE to go and find another software that does the same thing. And for a browser, there is relatively little UI to begin with - it's mostly an empty box to view websites. The technical folks are used to that, but the majority of market share is non technical people. The "be 10x better" paradigm is especially hard when it's your empty white box versus mine.
Thinking back, I suspect adblocking and pop-up blocking was a big part. The IE era web was extremely user hostile, with pop-up windows and pop-behind windows everywhere. And Firefox* offered a solution to it by blocking those, which I think was hard for Microsoft to follow due to their partnerships with major websites. Tabs were another, but I think that was less important.
* Then Mozilla, and Opera did it first