In my area, it is usually availability. When you have both men and women and players of different levels where you have to get legal pairings, it takes a larger roster than an at-level same gender team.
For example, while Mixed is just three courts so six players, you can't fill the line-up with just any six players available, it has to be the right combination of genders and levels. If you have an 8.0 team and everyone is 4.0 it is easier, but when you have three each of 4.0 men/women and then two 4.5/3.5 men/women pairs and one 3.5/4.5 man/woman pair and just four women available for a match and only one of them is a 4.0, you can end up not being able to field three courts if you don't have just the right men available.