Romance opens up a lot of dramatic possibilities. Will the two lovers end up together? Is there some hurdle they have to get over that is keeping them apart?

A loving dom and a sadistic one, for example, fighting over the same sub (who may or may not have a choice) can make for good reading.

Readers invest more emotion in stories where they care about the characters. If the story involves nothing more than sadism, then there's no one to root for, and there's no point in sympathizing with the victim because you know they're not going to get a happy ending (even though such stories can still be sexy).

If it's a story about a bdsm romance, I think it should have some suspense, or challenges for the charatcers to overcome, or else it's too dull.

Just my two cents.