You would have people showing up with tickets in hand that were purchased at different price points, and if they had to move their date and pay the difference, they'd have different prices to pay, and if one of them changed their dates at a different time than the other, then even another change in cost.
Then you'd have people spouting off "the cheapest time to go is mid-January when costs start at $104 per day," and they'd be wrong if prices fluctuated freely and people relying on that information would be confused and angry.
Unlike a Broadway ticket which is locked to one seat for one and only one particular performance, and thus, the selling price can fluctuate and lock that price down; Disney tickets, however, can be moved to different days, and if that cost keeps moving, it'd be... messy.