I like to think it is because steampunk associates tech (as represented in the neo-Victorian fashion) with elegance and social status. This association shows in all parts the aesthetic. I, myself dislike too much in lines of the mechanics and such. However, I have a design in the works for a revolver that "fires electrified bullets". Even that will be rather streamlined by comparison to some out there.
I blame Doc Brown.
Hmmm... Perhaps people are forgetting about the "punk" aspect? Someone might choose to use a flintlock, or other muzzle loading weapon as a base because that's all their character can get their hands on. Same for oversized "add-ons". Maybe they can't afford the newest, smallest, and lightest components, but still want something other than a simple projectile weapon.
As for something innovative and functional without being "gaudy", I wish I could build, or even draw, the Regnad-Sprague Model 1905 Gasfire Automatic Pistol from my CR&A (Calloway, Regnad, & Associates) stories. Unfortunately, I have neither the funds or facilities for the first, nor the talent for the second.
These should provide more SP weapon inspiration. They are Bavarian Christmas Demon Scaring guns. This is a must watch video: Gun Fun!