They can't pull it now because they don't have big enough marketshare.

Gee I wonder why that didn't work out.

Surely not because they weren't pure enough. The browser market was always so big that pure alternatives were never with any advantage: on the contrary, they had more issue because they catered to a loud minority that can always push for a "you are not pure enough" narrative. Because who push for that just doesn't care (rightfully) to be mainstream.

Thinking back at the actual history (and I lived it, having used Netscape Navigator, the Mozilla Suite and Phoenix 0.1), my view is that when Firefox was winning marketshare in Windows-land over IE, the release of Chrome and Google behind it basically killed (slowly) its momentum [1]. Even if the momentum was surely paid for by Google who was the default search engine in Firefox and paid millions to Mozilla for it.

So, to answer your question: Google is the reason it didn't work out.

[1] https://en.wikipedia.org/wiki/Usage_share_of_web_browsers#/m...