Explanation time:
William has iconic quotes that the fandom loves (ex: “I always come back”) though Asgore doesn’t have any (at least not ones that the community love). Plus Asgore is lead up to be this “big bad” but ends up being just a traumatized goat king being pushed aside and even killed by Flowey. Asgore doesnt get nearly as much screen time while William has more along with being a mysterious character with a lot of lore surrounding him. Something that Asgore doesn’t have
