An anti UFO ETH (Extraterrestrial Hypthesis) might have historical roots because from the get-go (circa 1947), UFOs (initially called 'flying discs' or 'flying saucers') were not viewed as a scientific issue or problem but rather one of national security - after all, unknown aerial objects were being tracked on radar and observed by credible witnesses including pilots and especially military pilots, invading the sovereign airspace of nations, and I do mean nations from around the world, not just the United States. This era was after all par of the beginnings of the Cold War, so unknown aerial objects would naturally be of intense concern to the governments of not only the superpowers but, in the era of this new nuclear weapons age, to all nations. Further, from the beginning, the 'flying discs' were taken as representing a technology, albeit terrestrial technology, probably Russian (unless you were Russian and then it was American technology).Though in the early days of the then termed 'flying discs' era, national security was the issue. To avoid overreaction on the part of the public ('the Russians are coming, the Russians are coming!), this being just a few years after the turmoil of WW II, the flying disc issue had to be downplayed. Softly, softly and the less said about the Russians the better - flying discs had to be, in public admissions anyway, hoaxes, hallucinations, misidentifications, anything but the Russians or Chinese (or in Russia and China anything but the Yanks).
Ultimately nobody ever really had to worry about the Russians, Chinese or Americans (or any other terrestrial nation for that matter). But that wasn't immediately apparent.
By the time the flying discs were becoming more obviously a scientific issue as official public and unofficial secret investigations started eliminating terrestrial explanations of the artificial kind (unknown but highly advanced and potentially secret Russian, Chinese, American, etc. aircraft), well, by then the subject was well and truly a part of the military complex and intelligence gathering agencies and not just in the USA, though in the USA one not only had the acknowledged U.S. Air Force flying disc investigation, but also the unacknowledged participation of the NSA (National Security Agency), FBI and CIA (as later Freedom of Information suits uncovered). The scientific community was pretty much left out in the cold. Your average Ivory Tower academics don't normally hold top secret security clearances nor are they knowledgeable about foreign military technology capabilities.
So by the time it became clear and publicly acknowledged that UFOs weren't a national security issue, well over two decades had passed, and since by then it was all a non-issue (the only issue that counted being national security), no wonder scientists (with few exceptions) didn't take up the UFO baton as the military and intelligence organizations bowed out - at least they bowed out publicly, but behind the scenes, well that's another story. While publicly and officially stating that UFOs have proved to be of no national security concern, prudence dictates you keep a behind-the-scenes; an unofficial eye on developments; on cases, on things - just in case. "Remember Pearl Harbor" - You don't want to be caught off-guard again!
But the public never did acknowledge the flying discs, or by now termed UFOs were a non-issue - it never wavered in its fascination for UFOs, especially once it became obvious UFOs weren't secret weapons of a terrestrial foreign power. By process of elimination, if UFOs had nothing to do with a terrestrial source, therefore, an extraterrestrial power must lie at the bottom of things - the UFO ETH was born and matured.
To recap, the UFO ETH only came to the fore when the UFO TH (terrestrial hypothesis) was found wanting by official military and other national security agencies. The scientific community was in the dark since UFOs weren't associated with being a scientific issue, but the great unwashed did see the issue as a science issue and the UFO ETH obviously rang a responsive chord. Why? Well they (the public) had been preconditioned into accepting the notion that extraterrestrial life was not only possible but here, and here and now.
Extraterrestrials (usually nasty) in novels and short stories and films and TV shows were part and parcel of the culture of the times, often because it was easy for writers and Hollywood to substitute an alien menace for the red menace of communism. The aliens invaded, but that was just a roundabout way of saying the Commies were going to invade if we didn't 'watch the skies'. Aliens, as substitutes, were also popular in the pulps before WW II, again as stand-ins for representing nasty foreign powers. Aliens were also used as a whipping boy to make social commentaries. Aliens were well established in the public consciousness.
So, left out in the cold by the official establishment, scientists hadn't a real clue about the nature of UFOs any more than the military or public did, but at least the military's jurisdiction - national security - had been eliminated as a possibility, so they now washed their hands of the problem (or so it seemed). The public however wanted answers; the military couldn't provide them other than UFOs posed no threat to national security, so the public turned to the scientists for answers. But the scientists were caught on the hop. All they knew apart from the now non-existent national security issue was that there was not only flying discs being identified as just hoaxes, hallucinations, or misidentifications (for public consumption as reassurance that the flying discs weren't Russian, etc.) but that there was this fringe element towards UFOs - the contactees and flying saucer cults which were loonier than just about anything going.
The scientists weren't really aware that the cultists and contactees and alleged (for public consumption) hoaxes, hallucinations, and misidentifications were the chaff and the haystack. They didn't know that there was wheat and a needle to be dealt with. So, the wishes of the great unwashed be damned, scientists weren't going to dirty their lab coats by entering murky waters dealing with the only UFO residue they knew of - cultists and contactees and hoaxes and misidentifications and hallucinations. So, under pressure, their only possible response was a quick with respect to the UFO ETH "it can't be therefore it isn't - now excuse me for I got better things to do that will better enhance my career prospects".
