Could be many reasons, but it may be that he doesn’t believe he can get sober (that he does not have the strength or will to overcome it) and does not want to acknowledge it, so he sees it as easier to say he wants a divorce, so that you are free of him without him having to deal with the pain of admitting to and apologizing for his failures and shame.
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https://www.quora.com/Why-does-he-want-a-divorce-over-me-wanting-him-sober/answer/Eric-Johnson-311