64 bit CPUs have been out for a while, but 64 bit Windows was only released in April 2005 (http://www.microsoft.com/presspass/press/2005/apr05/04-25Winx64LaunchPR.mspx). BE 10 was released in January 2005, so predates it. As BE11 was the first major version to be released after Windows x64, so it makes sense that it's the first to be supported.
Yes it sucks if you're a customer who's bought Windows x64, but that's the way it is.