I had voted for "Lack of a land-base route". While it's a large map and I cannot see how a land route would be possible other than tunnels connecting the two islands, nevermind the journey that will be crossing that distance, I feel this may be the issue. Once either side (Allies mostly) secures the middle icebergs, the opposing team may as well be out of luck for the remainder of the match.
While this can be attributed to lack of coordination among team players, I have to agree with NodGuy that Allies hold an advantage over the Soviets here. I understand it's about map control, as well as the benefits of capturing the neutral structures, but at times it feels hopeless for Soviets to make a comeback. Hinds get decimated by Rocket Soldiers, making that approach unfeasible alone, and Allied naval superiority rules out Soviet LST rushes. With those as the only two attack routes, it becomes a waiting game of the Allies moving off the icebergs and making a run at the Naval Yard and game over.