Since this seems to be my fault, allow me to elaborate....
I know a number of people in the publishing industry who think steampunk's an outgrowth of an aesthetics and costuming movement. (Cherie Priest said in a reading I was at that in the end, steampunk boils down to gears on hats. I think that was a joke). Me, I've always thought of it as an outgrowth of "The Difference Engine", so there's a lot of alternate history is steampunk, and alt-hist is a legitimate SF form. Certainly *I* enjoy it.
But, yeah, there's a LOT of bleed between fantasy and SF in steampunk (Hellloooo, Parasol Protectorate!). So calling it strictly SF is not exact.
However, this isn't my problem with steampunk. My problem is that buried inside it is the implicit criticism - almost a mocking - that says "See, the brass and steam of the Industrial Revolution didn't produce this fantastic world... who are you to think that the science of the Atomic Age would be any different?" What it reminds me of most are the comments of the creators of "The Venture Bros." (which I
love) that their characters live in the midst of a monument to failure, failure of the Space Age.
Yeah, boys and girls, it's the year 2000, and nobody has flying cars or a ticket to a Moon colony. Let alone real space travel or, hell, a robot butler or an alien pal.
The irony is that (IMHO) the subgenre got kicked off by an alt-historical piece about the one place the future DID deliver - beyond almost anybody's dreams - the computer.
Now, I'm not saying that this criticism is foremost in the minds of all the writers, and certainly not the fans. But this subtext isn't terribly difficult to uncover.
It's hardly like everyone's stopped writing "regular" sci-fi and is only writing steampunk now.
Yeah, but its success heavily influences publishers. It's a simple return on investment calculation. If steampunk is selling, say, 5 times better than "standard" SF, the editors are not gonna be trawling the slushpile for genetic engineering or the latest epic space opera. This even worse with fantasy vs. SF (which is a whole DIFFERENT topic *again*), and since you can dress up steampunk as fantasy - or at least, "not the future" - along with its historical romance trappings, it sprints to the head of the line.
This is not theoretical (though possibly anecdotal) - I know writers who can't get "straight" SF published.
Now, that said, I don't *hate* steampunk. As with most things, if it's done well, I'll enjoy it. Even as I scoff at the idea that Victorian science could have pulled off genetic engineering, even if they *did* know about Mendel.