Because by definition it requires three browser wendors with _complete_ implementation of the standart.
Looking at it this way we have no version of HTML complete.
Because they can't even agree on a database schema. HTML 5 is a total disaster zone.
Unless things have changed recently Mozilla and Microsoft are refusing to support the SQL lite definition. MS avoiding the open source in favor of there own SQL implementation and Mozilla going to for a super cookie hash table nosql thing.
Video is a disaster, audio the same, I'm sure theres more. I've given up and am only supporting iOS HTML 5 for the foreseeable future.