Well, for one thing, steampunk is trendy, and YA and romance tend to follow trends for crossover works.
(TBH, there's so much variety in romance that a lot of titles need
something to distinguish themselves from the others, and that usually tends to be the setting, which is often just a decorative backdrop for the romance. E.g. the notion of "wallpaper historicals" where the author doesn't do any research at all, but just does this imitative fake regency to jump on the bandwagon of more popular authors, much like the
cargo cult science described by Richard P. Feynman.)
As for the proliferation of female protagonists, I would guess that a steampunk setting tends to allow for more opportunities to showcase female characters in an otherwise retro-historical setting without having to work around a bunch of potential restrictions, as the tech could act as more of a societal leveler by opening up additional occupational roles for women (such as mechanic, difference engine programmer, airship pilot, etc.) which are less dependent on muscle strength for hard labour* which was otherwise kind of predominant at the time and the AH aspect wouldn't restrict the educational or societal acceptance as much.
So if you were an author who really liked your Holmesian Victoriana, but deplored the period-accurate sexism/racism/whatever, then with steampunk you can eat your cake and have it too with your clockwork detective-bot and his feisty inventor/literary agent who records the success/failure rate of the little steel-gray cogitating mechanisms† of her automaton for the penny dreadfuls.
Also, aviatrixes are awesome, especially when combined with airships. (My favourite episode ever of
Star Trek: Voyager is the one where they found Amelia Earhart, having been kidnapped by aliens. I always wondered what those aliens did with Elvis.)
* You kind of saw the same thing in a more negative manner IRL with the employment of children in factory work once the Industrial Revolution really got underway. And later, most of the wireless telegraph operators of what people like to call the Victorian Internet were indeed women.
† Okay, this is more like Poirotian Georgiana. But the point still holds.