The benefit needs to be worth the effort. Early on, in the days you refer to, browsers were often crappy and motivated switching often. I bounced between IE and FF a lot (and later chrome), because things were often broken/bad, or various websites I wanted to use didn't work in one or the other. However, in the last few+ years, that's not really true (in any way I notice) now.
Basically, I think the only improvements that can be made are incremental/marginal, and aren't enough to make switching browsers worth it. They're all pretty good now. So, I expect market share for browsers to change much more slowly now.
