Eh, I use D2D, GG and Impulse, too. Granted, I don't use them as often as Steam (as in, the majority of my purchases are on Steam), but sometimes there are some cool games and prices on non-steam DD services.
The truth is other digital distributors aren't trying. Yes, Steam is the best, but if you want to make a successful digital store, then you have to make it BETTER than Steam. Achievements, community features, in-game overlay, game tracking and so on. If you don't have these, then you'll always be #2 to Steam. Prices help out a lot, too. Sell every game at $1 less than Steam and see how that works for you.
"If you like it, they will stop making it" - Herblock's Law