I don't know that getting you out of the mindset of wanting to save the marriage is a bad thing. I have found it freeing to not be focused on saving my marriage. The sitch still [censored] and there are still waves of emotions, but it feels like a mental load has been removed. Really, she has to be the one who wants to save the marriage at some point in the future. We need to get to a place where we can take it or leave it.
As far as your question I would just tell her "because I didn't." Don't lie, just shut it down. She has no right to demand answers from you.
W 34 Me 42 Married 7 years together 8 0 kids 1 beloved dog BD 4/6/2018 I moved out 4/7/2018 I moved back in alone 8/05/2018 I file 3/06/2019 D official 5/7/2019