I’vre thought about this question a lot. I believe the answer is simple: because Halloween, regardless of its origin, is treated as a secular holiday and not as something that belongs to any particular group. So there’s not the same concern about offending anyone, meaning members of non-Christian religious groups. I think the biggest reason is that it can be wrung to the last drop in schools, with thematic tie-ins of every kind in the classroom, without triggering parents’ ire (in most parts of the country). I think this is why it gets built up to something so huge in kinds’ minds that the event itself is bound to be a letdown.
In the pre-PC days when I went to school, decades ago, Christmas and Easter got that kind of treatment, with no apologies to anyone. And they were correspondingly major in kids’ minds. Halloween was fun then, too, but never bigger than Christmas. In those days we also sang actual Christmas carols in school; and not just the sanitized, noncommittal songs like “Jingle Bells” and “We Wish You a Merry Christmas.” That too added to the sense of magnitude of the season.