…because Bane wasn’t portrayed in a similar way. Vader didn’t cry about Palpatine like Bane cried about Talia. Vader actually wanted to rule the galaxy. Bane didn’t actually have any goals for himself, he only cared about Talia. The entire film built up Bane: the self-righteous agent of judgement, and the finale gave us Bane: he-likes-a girl-and-that’s-his-entire-character.
I felt it gave Bane an instant humanization and a heap more depth as a character. Once that whole flashback scenes ends you go from looking at Bane as a back breaking beast to a dedicated and selfless guy. I don’t think it undermined his character but added to it.