Heavy armor gives high AC without any need to invest on DEX.
On top of that, in this game most high quality heavy armors come with additional damage mitigations ("absorbs X points of damage") when not even some significant buff ("Blade ward as a permanent cast").
That said, I do agree that it's the Medium Armors with no DEX bonus restriction that are complete bullshit. All the convenience and none of the drawbacks may sound awesome, but in reality this type of design ends up narrowing down your options to just one, because it becomes implicitly far better than anything else.
