Actually there is another thread almost exactly like this one... I believe it is probably under the restaurant heading.
To paraphrase all of the reasons given on that post:
too much overhead to pay to keep the cart to only sell 5 Beavertails a day.
they went from deep frying them to baking them and they weren't good, so the sales fell off resulting in #1
they went from preparing them onsite to going with a distributor and the quality wasn't as good, hence #1
The distributor was part of the Wendy's corporation and Disney deals with McDonald's, so the contract wasn't renewed.
I don't know it there really is a cut and dry reason... but if you miss them, or have heard so much about them and never got to try one, sign the above petition.... PLEASE!!!!