For each heaven, there was a limitation of power, generally defined by the laws of the world that dominated that heaven.
These are called primordial laws.
Even though they couldn't be felt by the overwhelming majority of humans and unlike the laws dominated by transcendents, they couldn't even be used directly, these laws were still there.
If a being whose power was stronger than these laws stepped into the world, these laws usually collapsed, along with everything around them.
That's why mortal grade was the highest grade of mortal heaven.
Of course, things of a higher degree could even exist, but when activated, they would cause damage to the primordial laws and when they caused damage to the primordial laws, it would result in one of the most well-known self-defense mechanisms of the heavens.
A tribulation.
Not like the nine heavenly tribulations that fell when someone tried to break through the skies and reach a higher heaven, but it was still powerful.
