Druids are kind of odd ducks. From a story point of view, casting them as a specific order of priests seems perfectly reasonable. Mechanically, though, they've always been somewhat different from clerics, and at least in 3.x had picked up so many abilities that they basically got chopped into three in the first spin on 4e druids (and while the Sentinel and Protector picked up some of the things that were spun out into the Shaman and Warden, they didn't get everything).