Kevin's comment about "why isn't science fiction ever set in the past, or an alternative present" reminded me of Dr. Grordbort's Infallable Aether Oscillators.
Science Fiction is usually set in the future because, by definition, it's about things that could be possible. Knowing that things didn't work out a certain way--such as Mars turning out to be a barren wasteland--put a damper on sci fi that requires us to remember what it was like before we found stuff like that out. War Of The Worlds, when Orson Welles made his infamous broadcast, scared the shirts off of a lot of people; today, such a prank would get barely a yawn at best, or a few snarky pedants going on social media to carp about "totally got the science wrong" at worst.
A friend of mine once said "Write science fiction, and the world will beat a path to your door to tell you what you got wrong." Where's the fun in that?
Steampunk is trying to fill the gap between what is, and what could have been. It takes the overwroght ornamental shimmer of the late Victorian and Edwardian ages and wraps it around nuclear physics, and throws in a healthy dose of verbose bulshitium. Peruse this catalog for the elegantly discriminating adventurer, who may find himself in need of some serious firepower for battling interstellar invaders or demons from an alternate dimension. Using the finest in phlogiston technology, you can revel in sociopathic glee as all of your enemy's molecules simultaneously decide to turn into soup.
And the best part is, no one's going to call you on your science, because it's all so over-the-top fantabulous that all you can do is hang on to your airship and enjoy the ride. So dust off your pith helmet, bone up on your best British accent, and come along on a journey into a parallel past where the hum of Dr. Grordbort's Unnatural Selector helped win the world for humanity.