This is a great write up on the history of IE.
Great to read all of the innovations that came from those times.
This is the money quote (for me at least) at the end of the article
The final nail in the coffin was that after the release of Internet Explorer 6, Microsoft decided to tightly couple new Internet Explorer releases to Windows releases. So they dismantled the Internet Explorer team and integrated it into the Windows product team
Somewhere in MS there is probably one or more people that made that choice. It was a REALLY bad choice. I remember at the time being an end user of IE and a computer science dude that I actively thought what a short sided dumb choice it was.
That choice and the group/person who made it singly killed IE. I am sure there are some other mitigating factors.