Vegito defused because the Potara fusion, when used by mortals, has a limited duration. It was initially thought to be permanent for everyone, but it was later revealed that its permanence only applies if at least one of the individuals performing the fusion is a Supreme Kai.
For all others, including Goku and Vegeta, the Potara fusion typically lasts for one hour. Since neither Goku nor Vegeta are Supreme Kais, their fusion into Vegito was subject to this time limit, which ultimately led to their separation.
