* Can't be blocked or monitored by state-controlled Internet
* Is widely installed throughout the population already
* Existence of which is not evidence of anti-government organizing.
That's what made AirDrop sharing so powerful for protestors. They all already had it, having it is not suspicious, and using it doesn't rely on government internet filters allowing it.
Protests have been around for far longer than AirDrop has existed. It seems AirDrop was being used to share posters and slogans and perhaps some basic information, not as some kind of instant messaging app. If Chinese iPhone users are anything like American iPhone users, I'd guess that most wouldn't want to receive a ton of iPhone notifications for unsolicited content.