I've been thinking about this. Mon Mothma is a non combat leader. Leia on the other hand... she can shoot, she is good on the front line as she leads by inspiring the troops with her.

Also, she allows a couple of honour guard spawns, who have infinite smart rockets, so there is that.

Another factor is that the rebels are a lot more cellular, so if you can hold a gun you tend to be holding a gun, rather than leading from an outpost somewhere. Major Derlin could kick ass pretty well, but still had to run management of a base. Compare to Imperial officers, who tend not to get their hands dirty.