Why are all Commanders robots?

Such a Federation thing to say.

Uhh... good one. it was in the sake of diversity...

Because robots have a fairly average movement type. They can go to most places, but not everywhere. Com balance would become extremely difficult if they had different movement types and speeds, given that expansion potential far outweighs other com characteristics.

Also, it would create the possibility of noobs screwing themselves over by taking the wrong movement type for their starting location.

How about All-terrain Amphibious Jumping Commander?
Or All-terrain Hover Commander that also fires anti-sub

That's all I got despite there is gunship commander

There was a proposal that I was a part of that presented a design we thought was fairly good, where you spawned with a very generic commander, and could choose a movement type via commander upgrade.

It was intended to make upgrading the commander more diverse instead of just more better with bigger guns all the time. I'm not sure that it solved all of the potential issues with movement type on commander, but it at least gave people the option, and since it was after the start of the game it allowed for some reactive choices rather than just hoping you picked whatever would turn out best at the start.

The issue, of course, is that it's a huge change, balancing it would be tough, and it requires art. So much art. Anarchid is the best point of contact if you're interested in it, or making something similar happen, but generally most people (I think?) seem to agree they'd prefer for commanders to remain generic robot movetype for now...