It hasn't alway been separate and different. It was once fully part of AB and was very busy and interesting. It was separated presumably to generate specific advertising and make more money.
Unfortunately the users hated it and it didn't work properly for a long time. A lot of the users gave up and went away, and the Sports site is very poorly used now.
Their have been many calls to re-incorporate it back into AB, but that would mean they would have to admit they made a mistake, so it never has been.