First, while I can't publicly state the actual numbers, I can tell you that there are many times more Visual Basic developers than C++ and C# developers. Therefore, basic business sense would indicate that - since it's not economically efficient to produce examples in all languages - we produce examples in the language used by the majority of developers using our platforms.
Second, we've spent a good deal of time and money evaluating how each group of programmers reacts to articles and presentations when the code is in the another language. These statistics show that the majority of C++ and C# developers are more apt to read an article that contains Visual Basic.NET material than the reverse. In other words, a very high percentage of Visual Basic developers will not read an article if it contains C++ or C#.
I'll leave trying to surmise why this is true to the reader. However, my personal opinion is that C++ and C# developers are much more accustomed to class libraries than Visual Basic developers. As a result, they can more easily look past the exact language semantics of a .NET example written in Visual Basic and focus instead on the .NET types/classes being used. Visual Basic developers - on the other hand - coming from a less object-oriented background of Visual Basic 6, already have the learning curve of trying to learn a new way of developing software without the added overhead of also having to convert the language syntax.