javier0889: It's because it has a certain "air" which seems to be quite similar to the BG experience.
I mean, Leliana
is Imoen and Morrigan
is Viconia.
The Real Time with Pause combat, the way you navigate through the world, the way quests work and even certain Bioware cliches are easily recognizable. Also, the "it's almost like a new BG!" thing was a huge marketing hook for Dragon Age.
I missed all the marketing (I don't really listen to gaming news...), and found the game while randomly browsing in a store. (I'm living under a rock :3 )
I had no idea what kind of game it was, I just wanted a new RPG to play with.
Since I had no idea it was called " BG successor", I wasn't even looking for BG in it at all.
After I heared about the BG successor thing, I started seeing the similarities, but before that, I didn't have BG in my mind at all.
Also: Since I only played BG2 for the first time this year, some similarities, like Imoen=Leliana, was not that obvious for me :)