See, the thing is that a surprising amount of people go to WDW with a limited time. Some can only go for 3 days ( for financial reasons or because they want to visit other Orlando attractions ) and some tend to skip over Epcot because "its too boring." Well, if they went with a limited schedule and saw that "Epcot is celebrating 25 years of excitement with amazing new rides, shows, parades, and fireworks in our most spectacular celebration ever!" then it would change their minds a bit about which parks to go to. It wouldn't take away from the other parks, but it would make Epcot's attendance peak a bit more.
The thing that made the 50th anniversary so popular was that it brought in the crowds because of great new rides. That's what people really want. People WILL NOT want to go just to get a few cheap pins. If you were the average tourist, which advertisement would interest you more? "Come to WDW for the YOAMD and get free pins and possibly can win a fancy hotel room!" or "Come to WDW and make a special trip to Epcot celebrating 25 years of excitement with all new rides and shows. Don't miss out!" Besides, TDS always celebrates its anniversary of its opening and it always brings in the crowds.