It seems top mma and boxers fight twice a year. But it seems the average boxer fights 5-6 fights each year while the average mma fighter only fights 2-3. It seems there is a higher probability of injury in mma that is why they fight less than boxers.
To be fair I'd say the average boxer fights 3-4 times a year. That said, the chance and risk of injury is much higher in MMA.