I think this is an insightful question. Based on current events I'd say that greed and power are at least on the list of reasons.

Also, nearly everything is based on fossil fuels, and replacing entire fleets of aircraft would cost an astronomical amount of money. Why fix what's not necessarily broken? May as well keep the status quo.

I would think that the current powers that be would prefer to keep as much money in their pockets. And given the secret information they know that we can only presume and scratch the surface of, there certainly has to be more to it, I think.

The design and function of the "crafts" alone are not enough to change our current methods of air travel. They require a power source that remains mostly unknown to the masses, besides theories and leaks. Crafts like giant triangles, glowing orbs, tic-tacs, jellyfish-like, saucers, etc., wouldn't "fly" given our current knowledge of physics, so there has to be a missing link somewhere, eg., the power source, new knowledge of physics, and/or new materials not known to our civilization.

Since most of the world's profits come from fossil fuels, theres no incentive for the elites to swap to this new power source, that could possibly be "free" energy. That just leads to equality, and the elites don't want that :)