Because if she sucks him she'll die
Nobody knows that much-- right now, they're all operating as to avoid risk.
When Kou went on his diatribe in the beginning of this chapter about how everyone acts like they're so smart for their self-satisfaction, I thought back to Hatsuka telling Nazuna about what happened when Kiku sucked Mahiru's blood-- an event he was nowhere near, involving a mechanism he didn't bother talking about until after he heard about the events third-hand.
Up to the Hokkaido trip, in fact, everyone except Kiku thought that a vampire sucking the blood of someone they loved would kill
the target, and that Kiku was intending to risk Mahiru's life in order to prove her love was real. Kiku, meanwhile, informed Mahiru that the same aforementioned circumstances would lead to the death of the
vampire doing the feeding. She also sought to do this as a means to die as a human.
Despite this, it's still unclear as to what happened to Kiku and Mahiru beyond the latter's vampirization, because while they both disintegrated, they did so in sunlight.
Everyone's playing it safe because of the severity of the unknowns, but-- outside of maybe Azami and Susuki-- they also overestimate what they understand, and it shows in the way they speak.