Most systems, as far as I know, stay mostly the same between editions. Yet Dungeons and Dragons has had major changes between almost every edition, with the skill systems, combat abilities/feats, multi-classing and monster stat blocks changing a lot, and features like THAC0, CR, and paragon paths being added and removed.
Do many systems do this, and I just don't know about them? If it's just Dungeons and Dragons, why?