If I were the suspicious type, I'd say that this COULD have something to do with the worry that passive cooling might not work quite as well as Disney and Dopplmayr hoped that it would. The parks are hurting right now under a rash of negative publicity re: price hikes and the mysterious drop in attendance this summer, and if they had any worries that all those Disney bloggers might come out with reports that the new gondolas are indeed uncomfortably hot; that would be a good reason to hold the announcement of the opening date until they had time to actually test in true summer weather conditions.
If the inside of those gondolas are turning out to be not so delightfully breezy after all, then stalling the opening until the worst of the summer heat has passed would be a proactive PR move at a time when they are trying desperately to manage negative press. That way, by the time next summer's hotel guests prepared to step aboard in real summer heat, all of the publicity surrounding the opening would have died down, and vloggers would not be bothering to talk about it so much anymore.
FWIW, I'm not naturally that suspicious -- I think it's probably a tangential contractual issue of some kind.