The main reason Microsoft is not pushing windows as a gaming os is because much of the techincal support problems it would get would be games incompatibility with certain drivers/hardware combos etc.Regardless of how many people buy this os for games,the majority buy in bulk for corporate use and expect to be able to contact tech support when they need it and not be on hold while someone spends time trying to get thier latest games software to run.
The fact is games DO run well on it in most cases but it's a simple cop out clause for both Games companies and Microsoft not to have to deal with games incompatibilities that are inherant with any new os but just stating it was never inteded to be a games platform so all games software is unsupported.