There were 2-3 reasons for eliminating Ethereal biomes temporarily:
Its easier to design for the more popular and standard default biomes. Most mods support the default biomes. Its easier to balance the game around this first.
Ethereal clears the default biomes when it is initialized. This means the default biomes don't show up at all when ethereal is running. This can of course be modified now.
Ethereal has a lot of items. Some overlap with magic stuff we already have.
Eventually, ethereal will be re-added in the future.