Actually,
you can use VB.net to develop WP apps. It's just that nobody does.
Honestly, there is no reason VB.net shouldn't die.
There aren't any important differences to C#. Both use the exact same CLR, both use the exact same .net libraries, and the compiler generates almost identical IL code. C# does support some additional language constructs compared to VB.net, but nobody is forcing you to use them. The only real difference between VB.net and C# is syntax. Anybody that can't get over that is in the wrong profession.