Based on what I've been reading, the reason is pretty simple.
Each case is different. Some are upgrading from Win 7 to Win 8, some have the Win 8 Media Pack, some are doing a fresh install or are running a new PC with Win 8.
Then there are different version of Win 8 and iTunes. Many are still running the 10.7 version of iTunes even though a newer version is out there. Win 8 RT is probably not going to work.
It's a **** shoot! Hopefully, once I get the Media Pack installed, my iTunes problem will be solved.