I already speculated on how the revelation in TL3 might have crushed Mami's self-image that she might have been upholding to keep her life meaningful. Realising that someday you'll be the very threat you opposed and that you've dragged three girls into this hell just because you didn't want to be lonely...
In contrast, there is no such danger in TL6. Under Madoka's control, the system isn't so different from how Mami viewed it originally.