I'm a big fan of the nu52 Harley, the character has grown past the Joker obsessed bimbo she was in the animated series. I can understand why people are baffled since the character doesn't fit into any set archetypes but the reason Harley is so popular is because a her book is fun to read it refreshing reading something more light and wacky, I mean have you read any of her nu52 series? I find most people who start these threads have never read the series.
I assume most people on this forum don't read her book. There's usually never any discussion about it or her most of the time so its fine for people to question why she's so popular.
Its like some parent wondering why his kids like Harry Potter or Hunger Games so much.
You get the sense that a lot of people had eyes on her at the start of the new 52 didn't like what they saw and then just gave up on her. Now we have a much more fun version of the character again and people are still hung up on her look.
There is a non-conformist strain to the millennial comic crew these days, though this is just a guess, I think her popularity is drawn from the same place that Deadpool's comes from. It might be something I because of my age can't see that has been tapped into or it could just be trendy. Ala Spider Gwen who was crazily popular before the character was even fleshed out yet. Maybe it is just a character design that captures the imagination. I remember Quesada's design for Azrael, I thought it was so cool in the free DC preview that I bought the mini and was therefore on board for the Knightfall saga from the beginning. Which lead through every issue of O'Neil's Azrael book. If I am completely honest if dealing with people who come into comics at a very early age why do we gravitate to the characters that we do? Sometimes it is the costume, sometimes it is the type of stories sci-fi as opposed to westerns as opposed to war books.
I read Suicide Squad for Harley until I reached my limit with her Nu52 characterization. I gave her new solo book a shot. So yeah, I did attempt to read her books. I just don't like them or Nu Harley.
Harley is proof positive that you really can't control which character become popular. Editorial has spent a lot of time and effort trying to sell fans on certain characters, but Harley was popular from the jump pretty much.