Publishers are more likely to put something out that conforms to their pre-existing prejudices. That said, E.L. James self-published at first iirc, so someone could write a book about a healthy BDSM relationship. There probably are books like that out there that simply aren't bestsellers because they don't have the hype. Also, underneath all the sex, E.L. James' book is an extremely traditional romance novel. Young and unexperienced woman meets rich older hot asshole, she mends his wounded soul as she is so Special and he "reforms". (In this case, stops being so dominant, because there is nothing E.L. James does not get wrong.)

Just for myself, I've never written anything but very short pieces that were focused on BDSM because, well... I can't imagine writing a whole novel revolving around sex. They're boring enough to read. Large amounts of sex and romance, yes, but 300+ pages of sex, yawn. I'd guess that there are people with healthy BDSM relationships who happen to have bestselling novels, but since their entire lives do not revolve around sex, they're not interested in writing characters whose entire lives revolve around sex either. I've been playing with how to write BDSM sex scenes in a novel that is not about BDSM, and it is not easy.