It can be priest or calliope.
1. They lack this type of weapon in stock to punish heavy static play in team game which leads to USF player have to pick the only two commander with Priest/Calliope everytime.
2. To promote diversify play style of USF and allow more strategic options in stock for them.
Flawed and useless argument like below are not welcomed because they are not constructive:
"Every faction has it's perks and cons, so USF can't have non doctrinal mobile artillery", the reason is describing the nature of asymmetrical design and simply isn't supporting the argument. Even asymmetrical design needs asymmetrical balance that's why this balance section is existed.