As it relates to divorce, “why” is not a question worth asking. There will simply never be a satisfactory answer.
There are those who ask what they contributed to the failure of their relationship, which is truly something we should all explore. But if you’re asking why your partner wanted to leave, you’re never going to truly, fully understand in a meaningful, satisfying way. As far as you’re concerned, there couldn’t possibly be a reason that’s good enough.