Major rewrite and redesign takes time even for the world largest software company, in the past 8 years or so both MSFT IE and Firefox has gone through major rewrites in their js engine, rendering engines and UI. Chrome on its part forked webkit so that they can make changes and add feature faster, which was not possible before.
Making browsing and maintaining your own rendering engine is not easy which is why we don't have a lot of competition in the this market.