Actually, I don't think Microsoft cares that much about their browser. It's not like it makes them any money. It was a selling point for their OS. Now there are plenty of alternative selling points.
What does IE mean?
1) Your clients need windows to run it.
2) Your developers need windows to test it.
3) They can plug in some of their technologies like .Net which runs on windows servers.
4) Clients that are used to IE (dumb users) will use it at home too because its "their browser".
5) Windows also means MS Office usually (though a separate issue).
So yea how is that not their core business model?