I'm going to put in my own perspective on steampunk here. Bear with me - it's been simmering for a while.
The old Flash Gordon type sf, as silly as it could be at times, was based on the mindset of the Age of Exploration, and that it would continue into the Solar System, and then into the stars. We got a lot of very good sf, much of it space opera, along those lines. And people believed it would happen and even worked to make it so.
The Solar System proved to be totally barren and hostile to unsupported life on a scale unmatched on earth. The sf continued on the grounds that we could terraform the other planets until we found our hyperdrive or jump points. And serious writers and readers had to accept the fact that FTL travel depended on the sort of handwavium of the cartoon here:
http://t2.gstatic.com/images?q=tbn:A...W79_W-CtigWQMw
But both transport and terraforming depended on cheap and efficient energy sources, which were also taken for granted. Atomic energy too cheap to meter and used for everything (Asimov). Fission (whose drawbacks Heinlein saw early on) replaced by fusion. (Heinlein.) Even using atmospheric electricity (Leinster.) Just like the settlement-friendly solar system, those power sources were not forthcoming.
Meanwhile, the technological revolution of the past few decades early on went off into either arcana (who today, if they're not in the trade, understands the chips in their cars or appliances? Let alone their computers? ) or a specialized and increasingly low-paid trade (see "Microserfs").
Steampunk technology is something we can understand. It's accessible to any child of the 20th century. Steampunk culture returns us to very first days of the techno-optimism cited above, when the British Empire ruled the waves, and if anything happened to it, America was waiting in the wings. In other words, it gives us a sense of a do-over. And on the cultural front, there seems to be a hunger for the dignity, dressing up, and stateliness of earlier periods.
Meanwhile, popular culture went off into vampires, werewolves, zombies, witches, devils on earth and angels fighting them, etc.... which makes sense if you consider them to be reflections of what they dimly see all around us in the culture. Wall street. Gangs. Hungry masses devouring brains. The bitchy boss. The people ruining it for everybody and the valiant members of one's own church or party standing against them, often using the same tactics. And so on.
Next to which, steampunk seems clean, simple and upbeat. Not to mention it puts one in contact with ways of doing things that do not depend on the grid, which is another popular underground theme. That's one of my own hobbies.