- They've already made a huge investment in the Trident engine which is now standards compliant and works fine.
- It would break backwards compatibility with existing websites since you'd have to recreate the issues in IE7 for its compatibility mode
- MSHTML is used by a lot of programs, introducing the CLR will introduce problems like if the program already uses the CLR, but uses a different and incompatible version to whatever "IE.NET" uses
- The NSAPI and ActiveX plugin systems are here to stay. Adobe and Apple aren't going to bend over backwards to make a managed interface for Flash and QuickTime respectively.