It’s never celebrated/observed on the same day unlike your birthday or Jesus’ birthday (I know it’s not really the 25th of December, but it’s always celebrated in the 25th).
People are just mourning Jesus’ death on a random day just because it’s Friday lol
My guess is because that's how it's always been done, and organized religion is kinda inherently conservative. Hard to see the Catholic Church changing anything like that for no reason