Mostly exercises are done early morning because we are filled with energy then and are empty stomach before the breakfast. Exercises need to be done on empty stomach or with very minimal food inside. Some exercises are done without having anything all. So, this happens early in the morning after we have a long food break at night.
Also, in the morning, we have fresh air, less polluted and the environment before the sunrise is the most suitable and is known to be harmless. During this time, people worship Gods and is termed as a pure time for doing exercises. Since, we generally, breathe a lot during exercises, we need pure environment and that is available at the morning time only.