Perhaps because Firefox doesn't support real-world web standards, has a laughable (and headache-inducing, with the default config file) rendering engine, and thus fails to render a great many web pages correctly?
Sure, Firefox is slowly copying many of the features long offered by Avant (as is everyone else... including Safari and Konqueror)... things like tabbing, pop-up and ad blocking, etc.
But Firefox doesn't even do those things well. It's pop-up blocker is worthless.
Unless Mozilla/Firefox cleans up its act, most users will continue to use the best browsers for their platform... IE (or a variant like Avant) on Windows, Safari on the Mac, and Konqueror on most anything else.