There aren't a lot of Windows 7 touch devices, and the ones that do exist have proprietary drivers since Win7 doesn't support touch. It doesn't make sense for Microsoft to spend a lot of time back-porting the whole touch system to Win7 and then pleading with OEMs to provide touch drivers there as well. Even assuming they were able to do both of those successfully, you'd be stuck using touch with Win7 desktop apps unless they back-ported Metro to Win7 as well.

Doesn't make a lot of sense once you write it all out.