They changed the rift a while back to make it so that the map was less intensive for computers to run and in doing so changed key aspects in a different update like changing the bush situations. If they reused the map skins from before it would look notably distorted and there would be bush issues.
"Why not change the maps then?" Their art team has gone through a handful of changes since then and bringing back those maps, even with slight alterations, would be like making a new skin for Master Yi but use his old model.
Riot has been working on the new rift that will roll out soon and, seeing all the effort going into it, I entirely understand them wanting to wait until that map is out to continue releasing map skins. I'm not saying that they will for sure, but doing so on the new rift would be a much better use of their resources than making a harrowing skin on a map that's being replaced shortly after. They could reuse that asset for future years and get more for the value than making a map for the current rift. Also, because of what Riot currently has on their plate, I would expect no map skins before Summer 2015, so we're unlikely to get a winter skin for the new rift nor a spring one should they decide to make future map skins.
And that comment about success inverting how much they give, they've basically (in the game's current state) replaced map skins with limited time game modes. Where the map skins were purely aesthetic and pleasant for the while they were around, we are instead getting more active changes to the game such as URF and other such modes to give the players more variety and engagement. It's not that they are giving less to their community, it's that they want what they do give to be of value and not just "wow factor".