It's as simple as it is overlooked: by building off of OSX, Apple delivered a phone that could actually work with all the files and protocols that people used on desktop. PDF, HTML, MP3/AAC, MP4/H264, ... Not to mention frameworks and APIs that were battle hardened.

It took competitors years to catch up, everyone's forgotten how much of a joke Android was until v4, and how Black Berry only caught up just as they were about to die. Things like low latency audio, large image support, GL, crypto, battery life, ...