Inductive charging on the AirPods case is trivial. Apple already ships a similarly sized inductive charging system for the Apple Watch, which has a significantly larger battery and slower charge rate than the AirPods case/headphones.

The more likely reason they aren't shipping this yet is that it was a companion piece to AirPower: it doesn't use the Qi standard.

I don't think that's the reason, you can buy a case that for the Airpods case that adds Qi charging. I would guess they were planning on releasing it at the same time as the Airpower mat.