They never became cool. The problem with the prequels are: they came out at a time of a whole new generation of fans. The newer generation won’t appreciate the original trilogy like we do and we see the prequels as a desperate attempt for George Lucas to make a little money.
The PT covered pretty much the majority of my childhood, and I’ve always preferred the OT. All of my friends at school did. I think we were too young to really care about old vs new. Granted the SE did make the OT seem much newer (I was the only kid that owned a copy of the OOT). But I don’t think that appreciation of the OT is the problem, and you definitely didn’t need to have experienced them in the 70’s/80’s to prefer them over the PT. Maybe it was unique to my experience, but my classmates enjoyed the OT and “old” films like the Indiana Jones movies just as much as we enjoyed current film series like the PT, LOTR, POTC, etc.