That's because it's a game I've always wanted to have maps for, either by me or someone else, and the fact that it's the black sheep of the Metroid franchise seemed to remove any enthusiasm from other potential mappers, thereby putting the responsibility on me to do it or no one else would.
As for Zero Mission, aren't there already two sets of (nearly identical) maps for it? It's just that none have been submitted to the site yet. You can check out Wileee and Edsword's map set at
http://www.screenshotmaps.com---
"With the first link, the chain is forged. The first speech censored, the first thought forbidden, the first freedom denied, chains us all irrevocably." [...] The first time any man's freedom is trodden on, we're all damaged. - Captain Jean-Luc Picard
Current projects:
(Gen), Sonic The Hedgehog (Gen), Sonic CD (Sega CD), Mega Man Zero (GBA), Battletoads (NES), Bucky O'Hare (NES)