(Hi I'm Penelope.)
It's mostly because we didn't want to punish the player for guessing right by having them miss a substantial amount of content in the final scene! This game was a jam entry, so we didn't have the time or resources to really make the interactivity more than a gimmick.
But even from an in-character reason, there's a very good reason why Leila waits till the very end to pick that option:
<spoilers below, everyone!>
...she's very much trying to rationalize away the truth/procrastinate a confrontation.