I'm guessing the people who liked it as adults didn't grow up reading lots of scifi so it seems more original and fresh to them. I've been a big scifi guy since I was in grade school. The book is full of tropes and they felt stale to me by the time I was graduating high school.

I haven't read Enders Game since about 1991. I still read through most of the Heinlein juveniles every few years although some have aged worse than others.