Due to the way WhatsApp does (person to person, not group) chat encryption, the number of messages scales linearly with the number of devices of either party to the conversation.

They probably want to put a cap on how much data/CPU the sending device has to expend per message.

Hmmm. Couldn’t they just use their group protocol for private chats, too?

I'm not too sure, but I suspect that due to how they do group vs. individual message encryption, this might decrease forward secrecy somewhat.