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".
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:
The Dark cards come in two rarities, common and uncommon. Of the uncommon cards 43 are U2, meaning that they appear twice on the uncommon print sheet. 35 cards are U1, these are usually dubbed the rares of the set. Of the 41 commons in The Dark 40 are C3. Maze of Ith is the only exception at C1 and is usually denoted as an uncommon although it is 50% more common than the U2 cards of the set. Also the common card Gaea's Touch comes in two versions, one has the mana symbols in the upper right corner of the card shifted more to the right than usual. For collector's purposes this means that one Gaea's Touch is a C1 (shifted mana symbols) and the other one C2 (normal).
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.