Truth be told, SOE hasn't had many games that sold all that quickly ...
Everquest built a large userbase over a very long time, and Everquest 2 struggled initially because it was released at the same time as World of Warcraft (which was widely considered the better game); and they have had a lot of games that simply failed to attract much of a market, like Planetside and Vanguard Online.
With that said, while I think DCUO is SOE's best game ever my opinion of the game is fairly mixed; and many of the things that I find less appealing to me are also the things that I suspect were put in place to make this game better suited to the PS3 and its audience.