Oz's castle acted as a scarecrow of sorts to other wizards in that area. Given his power, his presence meant either he would scare any others off, or those that were too foolish would have to face the magic that protected his castle even when he wasn't present.

When he vanished with the other wizards of Central, his protective magics vanished, and so did the lights. And even after Central's mission succeeded and their wizards were brought back, those lights didn't return.

The representative I spoke to at the ball shared most of this with me. They had fear of Central retaliation, as well as avalanches from the mountain, attacks from beasts, and wizards wanting to slide in to Oz's territory. So I told them to call on us, when they felt they were genuinely growing fearful.