Zenos is... [big fat spoily]

[Iscah]

I actually see it completely differently. Etheirys is what the Ancients called it when it was still their unsundered planet. We never knew our planet as anything but Hydaelyn and the Ancients are a thing of the past, their age is over.

So why should we suddenly adopt their name for it? Just because they told us? We don't use Gaia, Jöro, Ki, Geb or Tierra as names for our Earth just because people in the past called it that.

The simplest out-of-story reason is that it has always seemed unnecessarily confusing to have both Hydaelyn the planet and Hydaelyn the goddess, and it's easier to have separate names for them.