The lowest energy means to get to Mars is to use the Hohmann Transfer Orbit and the launch window comes about approximately every 2.1 years. Therefore, you’ll t

The lowest energy means to get to Mars is to use the Hohmann Transfer Orbit and the launch window comes about approximately every 2.1 years. Therefore, you’ll tend to see missions from different agencies all launch close together. More complex missions using gravity assists or relying on certain alignments etc are more variable in timing.