Given the pre-existing belief in angels and other celestial visitors, was it just inevitable that the emergence of air-travel as a viable phenomenon would give rise to the belief that creatures from outer space were coming here using advanced flight technology?
Or was there something about early-to-mid 20th Century politics or culture that could be jettisoned to make UFOs less of an attractive belief? I am taken to understand that Carl Jung, for one, thought that the belief in alien visitors was connected to the realization that, with the dawn of nuclear weapons, the mass extinction of humankind was indeed a possibility, and this put people in the mindset of pondering possible forms of supposedly higher existence in the heavens. Though most summaries of his views have seemed rather sketchy to me.
EDIT: When I say "interest", that also includes things like comic books and Hollywood movies portraying UFOs, even if the creators and most of the audience don't actually believe in them.