I haven't read any official articles/whitepapers as to why, so I'll venture a semi-educated(?) guess...
.NET was designed to bring in a new language with constructs/functionality/libraries to utilize modern day computing systems (64bit hardware, multi-processor/threading support) to compete with existing modern day languages.
They took a step back and abstracted everything into a common language so it doesn't matter if you code with C#, VB.Net. In the end you end up with an executable which provides with a lot of functionality out-of-the-box... you normally wouldn't have had (or had to do through various hacks).
Outsiders, like myself, would say Microsoft did a good job at duplicating/enhancing working-concepts of the Java framework and added what they felt was needed in a modern day language.
VB6 is old, just plain old. I don't want to fathom what type of effort it would take to revamp the core VB runtimes to match the functionality they have with the .NET framework today.