Physics, mostly. To hold the weight of any significant armor, fuel, materiel, etc. you would need an unbelievably large interior space surrounded by a relatively thin exterior.

Deployment is another issue - how do you get it to where it needs to be quickly?

Armor also isn't a good defense against modern missiles. The goal is to intercept or avoid getting hit in the first place, rather than sustain multiple hits.

I’m thinking more about a battlefield like Afghanistan, where the opponents aren’t firing anything larger than a stinger, and there is already air superiority. As buoyancy is a function of volume, which increases to the third power, and armor just needs to increase proportional to the surface area, which is the second power, a heavily armored floating aircraft carrier seems possible.

A 97000 ton aircraft carrier is equal in weight to the amount of air in a 450M x 450M x 450M cube. Aircraft carriers are only 320M long, so this doesn’t seem that far fetched.