From my experience, it is because of two things.
1.The rugged southeastern terrain and how few extractors there are there.
2.Oathsworn can't reliably get to Caer Mathos early game. Mainly due to the insane ZR choke point.
Basically, since the southeast is a wasteland, all of Dominion zergs to Caer Mathos. Since there is such little incentive to capture the southeast early in the game, Oathsworn heads north, and Nightfall fights a two front war from beginning to end, dooming them from the start.
The only way I see that Nightfall will ever have a chance is if population is reduced and strategy is introduced, or Oathsworn completely ignores Nightfall and goes south at the start of the match, giving them some chance of getting to Caer Mathos first.
Dominion and Oathsworn seem to have the best chances of winning since their front (in the southeastern "wasteland") is mostly small groups of people and not a massive zerg, but Dominion's access to Caer Mathos gives them that extra edge, that will more often than not, give them the win.