I think the reason is meta, Sunday is someone who refuses to vent to even his sister, he doesn't care about himself so the only way to know about him as a person is to read about him or listen to Robin. You have to actually like him despite his everything to go and check.
He's not gonna want to you to hate him less, if you hate he deserves it and he's fine with that so he dosen't need to try and ask for your sympathy.
I feel like once we get to his sp, his story will be more open.
@hxg_diluc i don't think there would be a cutscene since even trailblazer didn't get one of them actually receiving the ticket bc they had it on them since they first woke up (lol) but hopefully they would edit sunday's model sometime to include it since he's apart of the nameless now