1) Price increases are usually know FOR CERTAIN on the Friday prior to Sunday's release.
2) That is when all the new collaterals (pamphlets, displays, countertop price mats) come in.
3) WDW does not want to announce anything in advance.
4) People would buy the older and lesser priced tickets.
5) Thus, WDW would loss revenue.
6) Guests may not like it, but it does make sound business sense.