User blog comment:B214/High School DxD EX/@comment-201.239.133.207-20170104020556/@comment-1359322-20170105011915

Emotional attachment to the names aside. I think Ishibumi made the child inherit the mother's surname so that it doesn't confuse the readers. If every child has the surname Hyoudou i think everyone would be confuse as to who is who. This isn't like the main LN where you have time to slowly expand and let people know the character, there's a fix amount of space given to write this story.