It seems like Disney has just decided that 2 years is the proper time it takes to build a ride. Personally I think they are about correct on this. If you are coming for a 7 day Disney vacation, and you're the type that does it every couple years, you start planning a year or more in advance. If you are like me, you book 499 days out (and yes, part of that is the capacity problems at Fort Wilderness), but if you are staying on property it certainly pays to book at least 6 months out. If they can build a ride in a year, why do that? You want that lead time. You want people planning a year out to know what is coming to build the excitement, and you want to dangle that excitement for a period of time before people choose to book.
In other words, why pay extra to have it completed in 6-9 months when the significant booking boost you are likely to see won't be realized until 1.5-2 years. Sure you might get more AP and locals to come if you opened it faster, but they'll come anyway both before and after the new attraction happens. So it is really a timeline based on the occasional visitor and how to get them to book that next trip so they feel they are getting something new.