Because Visual Studio is 100 times more expensive, and not anywhere near 100 times as good.
And you worked that our *how* exactly?
Take a look at the following site : http://dotnet.sys-con.com/read/48835.htm you'll see the following pricing from Microsoft.
Visual Studio 2005 Professional Edition $799;
Visual Studio 2005 Standard Edition $299,
Visual Studio 2005 Express editions will be $49
Visual Studio Express, which is aimed at hobbyist programmers is $49, or actually $30 less than BlitzMax so I'm afraid your comment really doesn't hold up.