That is an interesting question. The only reasons that I knew of off the top of my head was that implementing multiple inheritance in a language adds a great deal of complexity to the language specification, the compiler, and the source code. Add the fact that multiple inheritance is rarely, if ever,
required to solve a particular challenge and there aren't a lot of reasons left to include it.
For questions like this I usually turn to the MSDN site, specifically blogs.msdn.com. There are a lot of blogs written on this site by the people involved in developing the C#, VB .Net, CLR, and the .Net framework itself. There is an article
here that lists a few more reasons for not including multiple inheritance in C#, VB or any other .Net language.