Because Disney has some sort of mythical, pixie dust laden cult status that it achieves in many people's minds as a result of being raised on The Wonderful World of Disney (for us oldies) or the Disney Channel and cutting their teeth on Disney movies and being hauled on annual indoctrination sessions known as "vacations." Mind you, I say this as someone who has taken 63 Disney cruises and who lives in Celebration, right next door to the Mouse, so I know what I speak of! The indoctrination sort of reminds me of the scene in Wall-E with the babies ("A is for the Axiom, B is for Buy N Large, your best friend). And if Disney is All That Is Good, then anything in competition must be The Root Of All Evil.
I'm just glad that although I'm a Disney fanatic, it's not to the exclusion of other things. I love love love Universal, and with my Premier pass I probably go there more often than WDW now. I hate to say it, but if I do a ride by ride comparison then USF/IOA pretty much kicks Disney's butt in my mind.
It's sad when people think it has to be one or the other and that they have to bash the competition rather than just acknowledging that both have positives and negatives (Gasp! I know...Disney has NO negatives and Universal has NO positives...sigh).