Because you still plant a seed in the earth, and you can still sell earthen wares. Lower-case earth still exists on Nirn, and that's what it's referring to. It's a forge deep in the earth, the soil and dirt and molten stone.

Aristocles22 wrote: »

Yeah, I've never understood that either. It should be "nirnen" wares. Like "nirnroot." They don't call it "earthroot." Maybe it's just an oversight. That, and "nirn forge" has a bit of a ring to it.

What? No. The planet is named after the dirt, not the other way around. The word for "earth" is older than the concept of planets. We called Earth like the ground we stand on. So it should not be "nirnen wares" and such because the name of the planet has nothing to do with it and Nirn was not named after its soil.
Nirnroot gets its name from the planet - not the element, so it would never have been called "earthroot". It's most likely named after the planet because the plant is so special compared to every other plant, so whoever named it probably thought it had some cosmic significance.