>>532014892
He got distracted by toriel and didn't show up at the church fountain like he was supposed to. There was nobody there to tard-wrangle the knight and people almost got hurt.

>>530781895
The sans doors anon. The sans doors. Sans and papyrus have been active the entire time. Only in chapter 4 does sans get distrscted by toriel, and that is when his doors don't show up.

>>530299291
Sans shirked on his "follow kris around" duties in chapter 4, he was too busy courting toriel. That's why the sans doors were gone.
If he keeps being too busy becoming kris' new dad the manipulator will need to swap in a new guy. That's where papyrus comes in.