Probably the same reason water doesn't escape over the edge of a bucket when you pick it up.
well, when you pick a bucket of water up, the water is held in because it can not pass through the walls of the bucket. if the ice wall is only 150 feet tall, that means that if you go up 150 feet, you run out of air, because any air above 150 feet would flow over the ice wall, because of the tendency of a gas to expand to fill all space available. acceleration keeps air down in the round earth model, the air does not have enough energy to escape earth's gravity, but it still tries to reach a state in which pressure is at an equilibrium, that is to say that the air moves across the surface of the earth in an effort to reach equilibrium. in the flat earth model, with the same acceleration, the air would do the same, but at 150 feet above the base of the ice wall, the air would still move laterally to eqalize pressure, but in doing so, it would move off the earth. i live at about six thousand feet. even if the government is lying to me and i actually live at sea level, i have traveled, on many occasions, one hundred and fifty feet up. i can verify this because from my starting point, i saw my final destination and it was certainly at least one hundred and fifty feet about my starting point. i could do some simple math to determine the exact change in elevation, but i'm sure most of you have had similar enough experiences to take this as truth