I think there's an allure of loveless characters, or characters who are obstinately against love, and this idea that *you* the viewer can fix them (or some other character can "fix" them through your fantasy).
For instance, April Ludgate in Parks & Rec has much stronger fan following online than Ann Perkins. Ann is beautiful, smart, kind of funny (well.. maybe not..), but April has always had more interest than Ann and i think it's primarily because Ann is so emotionally available in the show, while April isn't. So, not only do more people fixate on April than Ann, but also I think it makes the eventual payoff in relationships better. The payoff of Andy and April is great. But who really gives a shit who Ann is dating from season to season.
I suspect Kylo Ren shipping is similar. He's the dark mysterious character who spurns human connections and emotion, and that seems to naturally draw out people's interest there.