Note: this answer may contain minor spoilers.
Instead Momo (Lala's sister) becomes the primary female character.
What influenced the author to pull Lala from center stage and replace her with Momo?
I don't know how far along the anime is, but that's not entirely correct. Momo becomes a more important character than she was in previous TLR instances, but I think that's mainly for amusement / The Harem Plan™. And Lala isn't entirely dead yet. Why she isn't as prominent anymore? I have no real answer to that, but my guess is that her specific story is over and maybe the author just got tired of her. :)
Also, let's look at the name of the anime (and manga): To LOVE-Ru Darkness.
And whose name contains "Darkness" again? Right, Konjiki no Yami, aka Golden Darkness.
It becomes apparent pretty fast that she's the new main heroine, as the story revolves around her origin, her purpose, and friends / foes she has out in the universe.
A particular example would be Mea,
who is an artificial "transforming weapon" just like Yami,
and her mysterious master.
We also finally get to know Yami's mother. It's hard to see where exactly TLR:D is going yet, but it's not your average ecchi fanservice manga / anime anymore.