Because one definition of a derby is that it is the game against your nearest rival and so you can only have two, home and away, in a season. Sunderland already have theirs against Newcastle.
I note the Manc, Scouse and North London derbies are lower, probably some of the clubs involved have realistic expectations of trophies.
I suppose local bragging rights becomes less important in that context. The exception is Scotland, where the auld firm are the only competitors for trophies