UFOlogists love the idea of secret technological advancement because they think they deserved visions of 1950s scifi futurism being true. Basically blaming Jews for it all being hidden and occluded, etc, then it meshes with the other conspiracies talking about "the cabal" (antisemite term).

Most non conspiracy theorists have decided to most past those past futurisms as being largely dreamlike and unrealistic. But conspiracy theorists get a hold on people who felt like they were entitled to a greater future than was delivered and cannot accurately pin down why it is so. Not that their antisemitisms and other issues would be fixed, but there is a well-published essay explaining why the future is not "of flying cars" available that doesn't have to resort to the socialism of fools (antisemitism).
Graeber's essay is a good start for looking into that and critiquing the shortfalls in technological advancement and their economic and social causes, by critical understanding of capitalism. https://thebaffler.com/salvos/of-flying-cars-and-the-declining-rate-of-profit

However the "secret space program" and "secret technology" stuff will always remain a popular alternative as it appeals to old scifi tropes that captured the racist and antisemitic tropes of their time period and lifts them into big political players in our current reality. Reformulating fiction will always be more attractive to conspiracy bigots than assimilating new information because it contains older tropes that reaffirm existing prejudices. So you will see plenty of "draco reptilians" and evil Greys (stand-in for communists, Jews, etc) pitting battles against aliens that are tall white norse humanoids and more "racially pure" alien races that are coincidentally against "the cabal". Old scifi has a lot of historical baggage, and these people pilfer it for old timey bigotry escapism. Lots of what these people consume now are basically republished old scifi short stories being reinterpreted as "genuine disclosure" by "insiders" who coincidentally are all old enough and white enough to have read those stories in original print.