I'm a stan and I could attempt to explain... but when I try to explain why I like less popular characters in other fandoms it usually doesn't help much. What one person likes might be what puts off another.

I love Elle because precisely because she's so detached and snarky and not at all warm and personable. She's pretty much a stone-cold bitch all the time and I like that. But I also sensed there were deeper layers that we never really got to see because she wasn't with us for a very long time. As a fanfic writer, she's my favorite to write, because she's one of the most intriguing to try to unravel the layers and figure out what makes her tick.

I much prefer Hotch/Elle to Hotch/Prentiss b/c frankly, I just can't seem to see what everyone else is seeing. I wanted to like them because they're so popular and my girl isn't coming back but it just didn't happen. So I still hang onto my OTP b/c they're both so similar... they're not particularly open books, both deeply damaged but not one to let on, and both dealt with that damage in very different ways. I would love to see how they'd interact after the ordeal Hotch went through with Foyet. And yes, I have started writing that fic lol.