Breach for the Stars headcanon
Omega pups love lots of soft stuffs, they are hoarders of stuffed toys and plushie pillows. One is never enough for omegas and they know how to suck up to their parents to get more of what they want. Alpha pups, however, get attached to a single object. It may be a stuffed toy or a blanket or something else. Even if the thing is already in tatters or seriously broken up, they will never part with it and will treasure it forever. It’s not frowned upon that an Alpha apparently loves their favorite thing and keeps it secure in a safe. It’s quite common in the dynamics and it’s respected. If an Alpha doesn’t have a treasured object, then that’s a different story.
Sans has a lot of ponchos, capes and overcoats. Even though she’s usually seen wearing her iconic blue poncho, she usually switches based on her own mood.
However, Papyrus is like any typical Alpha who was enraptured by a single object once she was born. That happened to be the scarf which you used to wrap her in. You thought she would get attached to the whale plushie you got her, but nope, it had to be the cape. She outgrew her cape multiple times that you would eventually just sew a whole other cape underneath it just so she can still keep her treasured cape. Layers and layers of cape and yet she still wore it as her Sunday’s best.
Since Papyrus became the mascot of monsterkind, you decided to reutilize your Sansta cape and attached it to Papyrus’ original cape so that it won’t look weird to humans that she’s wearing something that already outgrew her.