I think the main driver is Library Fatigue.
If you're old like me, and were in college during the age of Napster/Limewire/iPods, you'll remember the intense enthusiasm people had about building their music libraries. We went from a situation where one you paid $10-15 per CD just to be able to listen to a couple songs you liked, to having basically every song in the world available for free. It made us giddy and irrational. You'd hear a Jackson Browne song you kinda liked, and you'd go to Limewire to download it. Then you'd think, "I may as well get the greatest hits colllection in case I like something else of his." Fine. But then you'd think "Well I might REALLY end up liking his stuff" and next thing you know you're downloading his entire anthology including Japan-only releases and half a dozen concert recordings. We were crazy. Then, once the novelty wore off, we weren't.
Same thing happened with the Steam sales. The combination of the burgeoning indie scene, people discovering Steam, and developers discovering that they could make a ton of money by selling their games for peanuts (but more than making it up on scale), led to frenzied atmosphere that drove mass over-consumption. And it's not like people didn't know what they were doing. "I'll never be able to play all these games LOL!" and similar statements were common. But there was a novelty and pleasure merely in being able to acquire so many games. People would brag about how they were going to spend beyond their means to buy games they'd admittedly never have time to play...
Then, suddenly, it wasn't so cool. These days we've got more games than we have time, they're available at great prices, and all that shit is no longer novel.
We have so many great games in our backlogs already that it feels wasteful to buy another game you're not going to play, and especially if you're not going to play it *now.* And frankly, if there's a game that's out now and I have an urge to play it over the other highly-regarded games currently in my backlog, why am I waiting to save a few bucks on a Steam Sale? If I'm that excited I should just pull the trigger now. Lord knows I've saved enough money to pay full (or near full) price by no longer throwing $2 and $5 at random games to sit in my backlog.