Bethesda have also released a statement as well, though their initial statement is "hooray for paid mods", it seem there's wasn't a clear agreement/timeline between Valve and Bethesda on pulling the idea
http://www.bethblog.com/2015/04/27/why-were-trying-paid-skyrim-mods-on-steam/
It's kinda pants they took it as "not a feature you want". It's a feature folks quite liked the idea of. It's the implementation where Bethesda get the largest 45% cut of the mod despite some (such as SkyUI) fixing their game. Seems Bethesda are maybe spitting thier dummy out that the little gold mine has faded into nothing. (Valve have stated their aim was to allow the paid mods to help fund the next gmod n such, but not sure how it planned to do that with only 25% going to the mod creator)
Still the way Valve word theirs they intend to bring it back, just to bake it into a game from launch rather than coming in 3 n a half years from launch and adding it in.
Valve have gotten halfway there with the likes of skins n hats n what not for CS:GO/TF2/Dota 2, but they're purely cosmetic n functionality baked right into the game. But mods for stuff like Skyrim is always along the lines of "make sure you've version 3.14 installed" n what not, and Steam has no way of running certain versions. And next to no proper refund system (maybe there's an idea they should test the waters with). Least with Dota 2 n the like it's a skin, you can see what it'll look like before buying. A mod could look fine on the screenshots but be shit in practice.
A donation system seems like a better idea, though could easily be put in place with a PayPal button or something on the mod creators homepage (which PayPal would only take 5% as payment processor, rather than the 30% Valve feel their entitled to for payment processing).