Because women die for Jon Bon Jovi and they're jealous. When I was in school, people would say "Bon Jovi is a band for faggots" "They're commercial shit" "Only girls listen to Bon Jovi". I never gave a crap, I just wasn't the kind of guy that would change his tastes to get people to like him more.
What's funny is that most of people that said those things to me, actually listen to Bon Jovi. The most metalhead guy in my school shared the other day "You Had Me From Hello" by Bon Jovi.
I don't know why is cool to listen to GnR and not to Bon Jovi. Maybe because Axl is a moron that makes wait his fans 3 hours under the rain, and has that though guy appearance, people might dig that. I don't know.
Lots of people that said that Bon Jovi was crap apologised to me after they listen to the whole These Days album. I honestly believe that they hate them because it's cool to hate them, and because they're jealous that Jon is hot and they're not.