^ Leadership is never decided based on a girl's popularity. It's decided first by the most senior generation (so it has to be someone in Gen 6, since that's the most senior one currently in MM), then by the most senior member, but in this case since Sayu and Reina have equal seniority, it's decided by age - and Sayu's older than Reina, so Sayu gets it.
And regardless, Sayu's probably the most well-known MM girl at the moment. She's the only one who seems to get any mainstream attention - even if it does happen to be negative a lot of the time
Personally, I'm looking forward to MM under her leadership, she and Reina will do an awesome job, I'm sure.
As for DMM, they're disbanding in a few months, actually. They were never meant to be a permanent thing, I guess.