That's a shame that they don't take voluntary banners anymore, but I can understand it too. They have a certain standard for their banners, so they were probably just overworked with the influx of banners that probably didn't meet said standards (and there are even more they have to deal with on the app for the app's exclusive banners and stuff).
I'd like to see them bring back voluntary banners sometime, maybe with more defined regulation/standards, but we'll just have to wait and see 