Why was the gas building up in the LEM but wasn't a problem in the command module?

The gas buildup during the Apollo 13 mission was a problem in both the CM and LM. The reason the Apollo 13 movie focuses on the LM is because that was where the crew most of their time, keeping an eye on the systems that were running and trying to stay warm. Also, because the CM was shut down, the makeshift LiOH canister adapter had to be attached to the LM Environmental Control System (ECS)