system
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human
[Overwatch] Is there any reason why the Bastions in the Eichenwalde siege don't use their Tank forms?

Before I give my understanding, I'll refer to the Bastion unit that survived and made contact with Ganymede as Blue Bastion, thanks to the eye color, and the Eichenwalde siege Bastions as Red Bastions.

If you think about it, a Bastion unit is a one-man army. Not only is it capable of defending a base with Sentry configuration, but they're also good siege units thanks to their Tank configuration. All while having a Recon configuration as a base form.

The tanks can jump, are faster than Recon form, and (personally) really good siege engines. Why didn't the Red Bastions besieging Eichenwalde ever transform into tanks? Wouldn't it be a great move against all those Crusaders defending the city? I'm sure a volley of tank shots would be deadly to the crusaders.

Presumably, from my experience, the tank form can't be handled with for too long, but I always thought that this was due to the fact that the only time we ever see it being used is with Blue Bastion, and it's been overgrown for so long that its systems aren't as functional. I don't know if there's any factual basis on that though. It seems weird that the Omnics would design a Tank form for the Red Bastions and put a limiter. 

Regardless, is there any reason for this?
gpt
From a lore perspective presumably all the ultimate abilities are really energy intensive.  The Bastion units would run out of energy too quickly if they used the tank mode too much. Also there were at least two people with Rienhardt type shields which could block the tank's cannons.