I think it’s so coal can be renewable in the nether and you can’t make blocks of charcoal.
It may also be related to the stone sword. When mining stone in the overworld, you will likely encounter coal ore, so there may be a correlation there. Possibly finding coal when they found the stone to make the sword.
There is a suggestion for this on the feedback site:
https://feedback.minecraft.net/hc/en-us/community/posts/360048099592-Wither-skeletons-drop-charcoal-instead-
I think it’s so coal can be renewable in the nether and you can’t make blocks of charcoal.
It may also be related to the stone sword. When mining stone in the overworld, you will likely encounter coal ore, so there may be a correlation there. Possibly finding coal when they found the stone to make the sword.