Muscles cells contain more mitochondria because they have to release large amount of energy quickly for movement.Energy is produced by respiration, the production of energy taking place in mitochondria. Muscles need alot of energy to contract, relax. So perhaps you can begin to understand why muscles need alot of mitochondria?
This is because mitochondria produce ATP during aerobic respiration and ATP is needed for muscle to contract. Without the mitochondria the muscle wouldn't be able to contract.