Oh wow. I don't even remember which map it is. I'll fire up the game real quick to refresh my memory.
Edit: Ah, this map. I want to say that people leave because it's mainly naval and air combat. If the Soviets aren't coordinated then the Allies can decimate them, because Longbows and Rocket Soldier combinations are deadly against Hinds and the submarines, and Gunboats absolutely destroy Missile Submarines. So if the former doesn't cover the Missile Submarines with Attack Submarines and/or Hinds then they're just easy targets. Because of this maybe Soviet players feel that they are unable to achieve victory so they leave? Maybe the Allies leave because they just want to captain Destroyers but have no defense against Attack Submarines, unless covered by allies.
Or maybe it is solely because there is no War Factory to pump out the vehicles players enjoy?