Apple has made the decision to disable the automatic playing of video on iOS devices, through both script and attribute implementations.
In Safari, on iOS (for all devices, including iPad), where the user may be on a cellular network and be charged per data unit, preload and auto-play are disabled. No data is loaded until the user initiates it. - Apple documentation.
Autoplay on most mobile platforms (Android, iOS) gets blocked to avoid poor user experiences - video should only play following a user action. You can usually work around it by triggering the play() on another event (eg the onloaded event)