The tradition (and, IMO, wholly unsatisfactory) explanation is that the spell is also restoring some of the fighter's "heroic luck," or "divine protection." So, since the 20th-level fighter is more infused with that than the 2nd-level fighter, it takes a mightier spell to fully restore him.
The real (gamist) reason is that, with spell slots, allowing an inconsequential resource like a 1st-level spell to heal 40 hp wreaks havoc with game balance. So they came up with the above (ridiculous) justification.
With spell slots being gone, I think Fourth Edition will dispatch this incongruence with a "bullet in the head."