Why is Hawaii time used for conference deadlines?
It's likely the most westward time zone with universities - or at least with a noticeably amount of academic work. Hawaii is in GMT-10, and in GMT-11 there are only a few small island countries, probably without any university.

In short, if you put the deadline in Hawaii time, in nearly any place of the world you can just forget the clock and just mind the date. It is also a great way to avoid complains like "I meet the deadline but I missed the time because I didn't notice the time zone", because in any other zone the deadline would be earlier.