The reason is simple:
The latest version of Mac OS X, Snow Leopard, may (or may not) lack some feature of OS X Lion required to run iTunes Match. Given that Snow Leopard runs both iTunes and the Mac App Store just fine, I'm not sure what such features would be, and it seems unlikely it will be a Lion-only feature on release (at least parts of iCloud, including the iTunes re-download support, certainly aren't).
The latest version of Windows – Windows 7 – is basically the same as Windows Vista, and both OS versions run all the same third party apps, including Apple's iTunes Match. If someone has a convenient and thought-provoking list of apps (not DRIVERS) that are Windows 7-exclusive (and not made by Microsoft), please let me know…
Long story short, it's not that Windows has a feature Mac OS X lacks, but rather that currently writing Vista and Windows 7 apps aren't two separate processes for developers… It's exaggeration to say Windows 7 is just a patch for Vista, but in terms of large-scale under the hood changes, there just aren't many.