I think three factors are at play (though not necessarily all three in every case):

1 - Some sci fi fans care very little about some things I care about a great deal (quality of prose or dialog, characters, that sort of thing) so will judge “great” a book, movie, or show that’s not just mediocre, but terrible at those things, because those readers/viewers aren’t tuned-in to those qualities.

2 - Many sci fi fans tend to over-praise works that aren’t bad, but also aren’t very impressive. I dunno if this is due to a low rate of really good sci fi creation, or what.

3 - Some genre fans don’t seem to read much outside their preferred genre, and may even hold one or another kind (I’ve seen multiple causes) of grudge against e.g. capital-L Literature, all with predictable results when they judge and communicate about works of their preferred genre.

(Clearly there may be some causal overlap between these, but I do think each likely occurs on its own, at least sometimes)

Fantasy can also be rough when it comes to separating wheat from chaff based on fan “takes”, but sci fi seems to have it the worst, for whatever reason.