Let me try and explain "it" for those that don't get it.
I've seen a lot of people that don't get it, it being Rocky Horror Picture Show. It's a tricky thing to try and explain to people. It's not a thing where you can point to and say "it's the music" or "it's the characters". Keep in mind this is very much my opinion, but I think my feelings are somewhat universal.
I think the reason it's so famous has got to do with the timeframe this movie came out. This was in 1975. This was pre internet. This was a movie that was talking directly to the 'weirdos'. These days they'd probably be the goths or the nerds or the geeks or whatever. Also, these days it's not so terrible to be a nerd. We've got Big Bang Theory and zillions of comic book movies and TV shows. But in 1975 "Rocky" not only embraced the weird; it also spoofed it and added sexuality to it.
To get an idea of Rocky you really need to see it with an audience. You need to see it with people dressed up. There will be people of all shapes and sizes, who often would feel not comfortable being in these sort of clothing or social situations now in revealing clothes. In a video I saw of one event a large man dressed as Frankenfurter attended and was having the time of his life.
This is all part of "Rocky". It's more than just the movie or the songs. It's the entire culture that surrounds it. It's the 'freak mecca'. People like me; who instead of playing basketball I was reading physics books or learning QBasic; could feel safe by watching "Rocky".
It was a secret club too You could met someone and you could say "Who's hotter Columbia Janet or Magenta?" If they could answer it told you a lot about the person. If they said "Actually probably Frankenfurter!" it probably told you even more! Tim Curry and his stupid sexy legs. :-)
The point is it's hard to try and judge this movie with modern day hindsight and try and "get it". We simply don't exist in that time now. We have instant communication on tap. Want to talk to 100 geeks about physics? Jump on reddit. Want a sense of belonging? Join a MMO.
We don't need RHPS like we used to. The need for a secret password to identify yourself as a member of the weird guys has gone. Now we're just part of the normal people. "Rocky" was a secret handshake between the oddballs, who now are partially the norm and can find other oddballs at the click of a mouse.
I still like Magenta though. She's delicious. Or Tim's sexy legs.