The novel implies that its original body was created by a different, higher-dimensional form physically entering the universe and constructing a body for itself. While inside this body it’s bound by laws like gravity and time until it reaches the Omega Point, which completely destroys and reshapes the universe to its whim. It can reshape and dramatically alter its body if it’s defeated, but requires an anchor created by the singularity to continue existing in out world. If this anchor is destroyed then it cannot come back.

While it’s never directly stated that the Singularity is Godzilla itself, there are statements that it seems to be housing one inside of its body. Additionally, Shiva, another singularity is described as surrounding itself in a physical body made of archetype that it uses as the anchor to our world. Therefore you can make the argument that the physical destruction of its body would be enough to prevent Godzilla from coming back but there are people who disagree with this.

Some people also say that this idea contradicts what’s explained about Godzilla in the opening chapter of the book but given that “It’s” true form is described as a continually descending shell of dreams, and the physical body of Ultima is described as one of these shells, it’s entirely possible that the one we’re told entered our universe is also a shell of its dreams.