Thinking about it some, the way things are going does make a lot of sense. The earlier Steam sales were likely not just about encouraging people to buy more, but also raising awareness of Steam and getting more people to use it. The fantastic deals and minigame sorts of things was great for spreading word-of-mouth. Now that Steam has gotten as big as it has and controls such a large share of the PC gaming market, they can sit back some and don't have to offer as mindblowing of deals or as exciting of sales.
Meanwhile, we're starting to see retails like Amazon taking Steam's initial approach by offering fantastic deals instead.
Yep. And keep in mind not everyone knows about places like GG or Amazon and such. Moreover, Steam rakes in the money from F2P microtransaction -- TF2 and Dota 2 are HUGE revenue generators for them. They don't even have to design the stuff. They let users do it and give them a percentage of the sales.