The suits are granted to the avatars through the power of their respective gods. However, there has to be a willingness to wear the suit. We see multiple times that the suit doesn't just come to the avatar and that the avatar has to willingly summon it. This is made most clear in episode 2 when Steven needs to summon the suit and doesn't know how to, Khonshu is unable to summon it for him.
Lastly, it is pointed out in episode 2 that Khonshu's avatar's armour comes from Khonshu's temple. It's possible that when Ammit was imprisoned and banished, her temple was also destroyed and that any ceremonial armour she did have for her avatars was destroyed; or the ability to summon it was lost. 781b155fdc