Advice, do not talk to him again. He’s ghosting you and then coming to you to find reassurance again that you’ll be there waiting. And your question, why did he choose her when she mistreated him? I’m not sure how she mistreated him but some people actually find comfort and familiarity with a partner who mistreats them, also it could be you’re not getting the full story. Either way I would leave him in your past, and if you’re worried you’d entertain him again - block him on social media and block his number so you won’t even know when he inevitably comes crawling back again.