Quote (Jays @ 22 Mar 2015 18:06)
Because it's a huge stadium they can fill, and it's somewhat equidistant from Barcelona and Bilbao (thus making it equally easy/hard for both fanbases to travel).
There's obviously other big stadia around, but the reason they want it to be held in Madrid is (officially) becuase it's not too close to either of Barcelona and Bilbao (like Valencia, Donosti, etc), while not being too far (like Sevilla would be).
The last time they played each other it was held in Vicente Calderón, iirc, which is big enough and in Madrid, so that could be a good place to host it.