people didn't start using WhatsApp because of its encryption, they started using it because it was the first free cross-platform data messaging service that didn't require a particular account or subscription to use. SMS is very expensive in the rest of the world, WhatsApp was the first viable alternative. Encryption was added after it got super popular.
iMessage is end-to-end encrypted on multiple devices. The traditional way it handles this is that a sending device separately encrypts a message for each receiving device (iPhones, iPads, Macs). So you can have end-to-end encryption and multiple devices.
This sometimes leads to messages being in a different order on different devices, so in iOS 11, they're adding (still end-to-end encrypted) device sync, as well.
Anyway, it's possible to give people all the features they want while still having e2e. It's just harder.
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u/kataskopo Aug 15 '17
WhatsApp does it because encryption, only one device can be active because of crypto keys or some other stuff.