It's just people being cheap. Insert any talk about "x" game on steam and there will always be someone who responds with "waiting for 75-90% off" jargon.
It's generally me. I think that D:OS was the first game I spent more than $15 for in at least a year. Got it for what it was during the Steam sale, $33 or whatever. Generally speaking, with the quality and quantity of both games and sales, there's hardly ever a reason, let alone time, to buy a game at full price. It isn't being cheap, just practical.
That being said, D:OS was the best gaming money I've spent in years. I was hesitant to pick it up during the summer sale just because of what else I could've gotten for that much money (I think I was eyeing Broforce, Volgarr, Mercenary Kings, and one other I can't remember for
less combined than D:OS), but it was totally worth it.