Just because you don't know them doesn't mean they're random.
Fuuko and Miru have been heavily pushed for the past year (Miru's popularity actually exploded this year).
And it's about time NMB used someone else as center. If this year's election showed something is that Sayanee is waaaaaaaaaay ahead of everyone else in NMB popularity-wise, and they need to shorten that gap. They don't want another AKB sans Acchan situation.