You want reasons?
Let's see... It does not work with many mpeg1 and mpeg4 codecs, does not play mpeg2 unless you pay extra, no full screen unless you pay even more, nagging screens to go Pro, it tries to preload as soon as I login (along with other such programs, this slows down your login considerably and if you disable these startup items through msconfig, then MS will display annoying warning messages at each login), does not play DVD's or VCD's, so you need one more program... Need more reasons? The only thing going for QT so far is the nice semi transparent white control buttons (pause, FF, Reverse etc.) in full screen mode. Other than that WMP on PC is much better.