Because there is no Pro version of QuickTime Player X at this time. And you're not "reverting" to QuickTime 7 as such, you're just installing QuickTime Player 7.
And if QuickTime Player 7 isn't already in your Utilities folder, you don't need to reinstall Snow Leopard. You can install QT Player 7 separately. Insert the Snow Leopard install disk, go to the Optional Installs folder and run the Optional Installs.mpkg. You'll be able to select to install QuickTime 7 (it actually installs only the Player) from there.
If you don't have your Snow Leopard install disk handy, you can download QuickTime 7 Player for Snow Leopard here:
http://support.apple.com/kb/DL923
Regards.