I am not sure why Carnival still blocks WiFi calling at this point. The speculation I've read is because they want people to use the in-cabin phone, or Cellular at Sea roaming, presumably they get a spiff from those fees. But I don't have any proof of that.
Internet and phone service is a huge missing link for the work from anywhere crowd, and it's a market that Carnival is entirely leaving on the table for their competitors. Their choice.