Because hypothesis is too grandiose a word for most people. They also don’t know what a theory actually is.

Because two syllables good, four syllables bad?

Ancient Astronaut Hypothesis sounds just as good

Probably due to conspiracy nutters not knowing the difference between a Theory and Hypothesis.

Maybe because they usually are facts someone in the know leaked but will get confirmed only later

Because laypeople don’t know the difference, or that the word “hypothesis” even exists and just use “theory” instead.

It doesn’t help that they don’t understand the word “theory”, and that the “theory of gravitation” or “theory of evolution” are not hypotheses…

Because people don’t use scientific language in day-to-day life.