I'm posing this question in this manner to avoid phrasing it "why the heck is it a $20 card if it's so common".
- Anybody can ask a question
- Anybody can answer
- The best answers are voted up and rise to the top
And by the way, the price tag might have more to do with the fact that it's an old, not-common, and pretty good card.
The comments and Wiki article on The Dark sheds some light on the matter:
Going by the checklist provided in the comments, The Dark has no rares designated as such, so all cards go up in rarity in proportion to fit today's C/U/R pattern.
As far as I can tell, all C3 remain Commons, C1 (Maze of Ith) and all U2 are Uncommons, and all U1 are Rares, by today's rating.