It is most likely due to time and effort.
Sure, they might be able to change it with little effort, but even then it would take a decent amount of time to pull off, due to the duration of any required QA cycles, which are mandatory unless you are willing to risk the Butcher's Bridge floating over the Howling Abyss background or any other glitch.
Now, the fact that they can do it doesn't mean that they should do it right now; It all comes down to priorities.
The new client is under construction right now, and the plan is to release it during the first part of the year. Any work done into the actual client would delay the release of the new one. Unless you are talking about features that provide a huge value (like the new Dynamic Queue, or the new report interface), it is probably not worth the trouble.
And let's face it, hooking up variant maps for the ARAM queue is very low priority in that regard, since is a cosmetic change, affects only a small subset of players, and does nothing aside from looking awesome (although damn, it looks really awesome).