I think the introduction to Michael Moorcock’s review of The Manual Of Detection by Jedediah Berry is worth quoting from extensively:
If rural nostalgia fuels the continuing appeal of Trollope or Tolkien, then its urban equivalent is most commonly found in Dickens pastiches such as Philip Pullman’s Ruby in the Smoke, in Holly Black’s gritty fairy stories and in the steampunk genre. These days, you can barely pick up a speculative fantasy without finding a zeppelin or a steam-robot on the cover. Containing few punks and a good many posh ladies and gents, most of these stories are better described as steam operas. The Manual of Detection formalises many of the genre’s themes and includes a dash of cyberpunk noir.
Cyberpunks were what the likes of Bruce Sterling and William Gibson called themselves when first signalling their break with conventional SF. What identified cyberpunk was a sophisticated interest in current events, a guess that the Pacific Rim might soon become the centre of world politics, a keen curiosity about the possibilities of post-PC international culture and a love of noir detective fiction. Characteristically, cyberpunk revived the noir thriller and might as easily be considered a development of the mystery as of science fiction. Sterling and Gibson’s The Difference Engine was an early example of cyberpunk merging into steampunk, proposing a Victorian world with Babbage computers and airships. Airships also appear in The Golden Compass and Watchmen, among other recent movies: they signify you are in an alternate reality.
Steampunk reached its final burst of brilliant deliquescence with Pynchon’s Against the Day and his Airship Boys. Once the wide world gets hold of an idea, however, it can only survive through knowing irony. Its tools, its icons, its angle of attack are absorbed into the cultural mainstream. The genre has started to write about itself, the way Cat Ballou or Blazing Saddles addressed the western. Steampunk no longer examines context and history but now looks ironically at its own roots, tropes and cliches.