At the time, and perhaps still today but greatly lessened, Microsoft was used to both becoming the victor by crushing the opposition and then staying the victor because no one dared take a second swing. Anyone else who thought about challenging usually thought against and then went and carved out a quiet niche elsewhere.

Besides, nobody got promoted at Microsoft for staying the course and making incremental improvements. Once there were no more balls to be knocked out of the park, developers moved elsewhere to buff up reviews and get bullet points for major projects.

IE6 was wrapping up development when the US government sued to break up the whole company for giving it away with Windows. Perhaps not a coincidence they stopped work then?