Well, I'm not sure I should get started on this but...
When you talk about themes and messages, which I agree is the biggest reason for both shows' success, it has to be highly cultivated to the individual. Different people relate to different things, and find their own situations mirroring different ones.
Not that the content apart from themes and messages is really that comparable, but yes I'll agree Rent deals with a more mature content.
But I'm not sure I'd call the themes and messages more mature. I love both shows, and take strong messages out of both.
The heart of Wicked to me isn't about relating to Elphaba as a misfit, or realizing that it's okay to be a misfit, though many probably do consider it that. To me, the whole idea is "Defying Gravity." Although Elphaba actually does fly, it even serves as a metaphor in the show, and one I've applied to my own life many times. Whether or not she's a misfit...the decision she makes to not go along with the Wizard is really amazing. I mean like Glinda says "who could resist this?"
That's what I view as the impossible thing she's defying. Making the hard choice. Deciding to do something that she realizes will make her life very difficult, etc. That's what I relate to. I started doing this during a really difficult time and every so often I still do--when I'm really scared to do something, or I have to do something I don't want to do--I sing Defying Gravity (very badly of course but with luck not too many people have to hear me). And it really helped me through my difficult time.
And obviously there's amazing themes and messages in RENT--the thought of measuring your life in love...so simple, but...just so right.
Really, individuals can find important themes/messages within anything. I once found a huge essay online that described these sort of themes/messages in Muppet Babies, and how thoughts of that show helped them through life. Hey, whatever works.
I think what may account for the more "crazies" in the Wicked fandom is simply that the show appeals to a slightly younger audience than RENT does. Kids at younger ages are seeing Wicked than RENT. So they're a bit more immature, and therefore a bit more "crazy."
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli