Because how Rena behaves and how the quints behave. Of course it just speculation, but for Ichika it would be least amount of bullshit to switch from her normal character to play Rena.
My order of likeliness of being Rena:
1) Ichika: Doesn't show up much in the chapters before and can easily switch roles as an actress.
2) Miku: Does the impersonation best and is known to be a stalker. Could be her as well but it doesn't feel right with her behavior before to pull that Rena stunt at that time.
3) Itsuki: Could do it in theory but would be inconsistent with her behavior before.
4) Yotsuba: Seems to be to honest to something so convoluted.
5) Nino: Her being Rena would feel like split personality. Saying goodbye as Rena and then inviting him into the apartment in one chapter.