…gain user traction faster than HEIF.
In less than one year from introduction, about a half billion iOS 11 users† are using it as their default photo format. Not bad for traction.
Of course it wasn't their decision, it just sort of happened.
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† I must confess I made up this number. >80% of iOS users are using iOS 11, last year there were 700M iPhones, there are some iPads too, and some Macs, though not all iOS hardware can do HEIF. Let's just say I'm probably within a factor two.