I think the short story is fabulous medium for spanking. My Cariad thought he'd got interest in my collections of stories only for the publisher to decide they wanted short novels after all.
I know that the publishers are supposed to know their market, but there seems to be danger of formulaic thinking. May be safe but reminds me of expression 'If you do what you always do, you'll get what you always get'.
I've read some good spanking novels, but all one can do in a novel is up the ante, and personally I don't find that especially sexy. The subby journey is interesting, but not the only way to go. Spankos may get more sub/Dom(me)or not. Like others may or may not turn into swingers or whatever.
For me there is excitement around first experiences and this can be done in hundreds of ways and is suited to short story format.
Also such literature is used to enhance and supplement fantasy and the short story is perfect fodder for fantasy.
I guess I'll have to bite the bullet and consider writing something to fit perceived market or try self-publishing. Ho hum...