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.