I too agree that Disney has probably done way with the shorter races due to lower attendance. I've done the ToT and EC but the costs for airfare, hotel, etc, make it cost prohibitive to return year after year. Likewise, I would probably never consider flying down to Orlando just for a 5k race, especially when I consider that on any given weekend from the beginning of spring through the beginning of winter I have my choice of several 5ks and often 10ks here at home.
On the other hand, hubby and I have been doing the January races for over 11 years and I've done both Princesses. Combine this with the fact the half marathon is now the most popular racing distance after the 5k, here in the states.
I think Disney is simply catering to the "demands" of the racing community and offerring them what the majority is looking for.