What is the primary difficulty in developing a web browser?

- breadth of the http/css/js standard? - inefficient implementations - requires too many resources?

Why has the market converged on two major players and most independent attempts fall short?

They're 30+ million lines of code to cover the feature set, but monetization is tough and security/feature churn is constant. It's also nearly meaningless to start small - either you build everything or you don't.

This is just Chromium with some patches though, the problem with these kinds of things is it's small groups that tend to lose interest.