While Valve is absolutely right, I think it's fair that for a lot of these independent developers, putting in the "lot of work" to get featured in the sale is a large part of what helped them maintain their long-tail so well. They have way more data than I do, but I'd be surprised if the regular participants weren't getting a huge boost from those promotions.
However, I suspect the coordination of those events on the Valve side of things was too much work for the very few product / platform managers they have. Scheduling sale programming (ie which games go where with where other games) is hard enough as-is, actually making sure all the devs hit the deadlines and stuff must be a real pain.
Also it strikes me that Valve has always made sure that Indies have a slice of the pie, but never the full pie. I mean, yeah, they've done a few small indie-only sales, but the vast majority of sales have a bunch of AAA headliners and a couple of indies. My negative tone here is because I think that's unfortunate. But events which require developer work are basically only possible for indies, not mega-devs... Infinity Ward isn't going to add stuff to last year's Call of Duty for a promotion. Even harder for games that lost money for publishers... why would Take 2 pay the dev to add stuff to Spec Ops?
Honestly I'm not sure why Valve doesn't monetize community participation more. As I mentioned with Greenlight, they should be giving me... say, a buck of credit for every 100 votes. Or a ticket for a wishlist draw with each vote. It's all well and good to have badges or summer achievements for "writing a recommendation", but why not encourage continued use of the feature by giving credit or tickets for a wishlist draw for each written recommendation? Why not add purchase links to screenshots or recommendations and give users a kick-back for successful click-through purchases? I think you'd be able to set up requirements such that the reward is low enough that it can't effectively be gained but high enough that people who are aware of the features but don't bother would start bothering. *shrugs*