>>96761722
>Lacking the manoeuvrability of other battlesuits, the KX139 Ta'unar Supremacy Armour was designed for static defence, carrying massively destructive, long range weapons.
This is the important part.

It's a giant gun that's used mostly statically. It's basically a slightly mobile turret. It might be able to blow up big things, but the reason it itself is big is because it's not supposed to be used in offensive warfare. They're not sending it to take worlds, they waddle it over to where they want long-range firepower.