Not sure why you're being downvoted, and I've no direct insight into Apple's deployment decisions. But I'd guess they're concerned that deploying MPTCP could c

Not sure why you're being downvoted, and I've no direct insight into Apple's deployment decisions. But I'd guess they're concerned that deploying MPTCP could cause problems with middleboxes in some environments. Siri is a good way to test the water. First, they own the servers, so they can ensure there's no problem on their end. Second, Siri isn't mission-critical in the way IMAP or HTTP would be, so if there is a problem, it's less of a PR disaster for them. Personally, I'd also prefer to see an API that developers can use to enable it, but I'm OK with them taking it slow at first. Every cellular and WiFi network on the planet got to see how their middleboxes handle MPTCP, so having them test the water like this makes life so much easier for anyone else later.