Folks usually lean one way or the other because typically readers looking for romance aren't looking for anything extremely dark. They're reading for the enjoyment of a fun romantic romp, not a psychologically challenging thing to break their minds or get them to think in that way. And folks delving into dark horror/thriller stuff only care for romance in however it might help along plot or characters but it's not the main focus. Readers have tones they like, and normally such rigid tones are more likely to float to the top.
That is not to say some folks don't dabble in both, or write less conventional thing. Just the usual suspects are more common and typically more "widely enjoyed".
Personally, I like to think I write a variety of things, but I identify most with being a horror writer. I've been told my stuff is too gratuitous and grotesque. So I suppose I do fall more on the latter side of your outlined divide, but that said, it isn't as if I don't try other genres. I used to write mostly romance before I moved out and started unpacking abuse as a possible subject to write about. I'm still dabbling in it currently with a swashbuckler side project. But I'm also a nobody, so. Not exactly the type to paint a platform in a specific light.