I think there's two main reasons, from my point of view:
1) I don't want to listen to conversations for 10 hours
2) it's a bandwidth hog, and would prevent other passengers from having decent connectivity.

I wonder if the FAA may invoke a security clause here as well. Even though the calls are not over a cellular network, I could see them having concerns with people communicating during flight.. although nothing is stopping them from using a chat/messenger service.