Oberon as the base made it better mechanically in my option - because of Oberon we have Warforged and the myriad of awesome build possibilities they represent, as well as dragonmarks, etc.
The game currently has both Forgotten Realms and Ravenloft settings as well, so its not like you don't have tons of content outside of Oberon. FR is like 25% of the game, if not more.
Them adding other settings was so popular, the ultra-craptastic Neverwinter Online followed suite and copied them.
But as for why DDO is still number 1 in my book - no other mmo even comes close to character building possibilities. In this regard DDO is in a league of its own. Am I a huge fan of everything it has and does mechanics or systems wise? No, not at all. But the chardev is so good it makes the rest of it acceptable.
I play on (I think) Kyber server. I'm not good at remembering server names so I could be way off. Kyber pops into my head though. I know my server is very populated. Too much for my taste.