Hawaii is not as unique as you think. Clinton's and Trump's shares of eligible voters were both only in the high 20% range:  44% of US eligible voters, ... didn't. 

Multiple reasons not to vote have nothing to do with mail in convenience. 1) Electoral college means HI vote matters little; 2) many think not registering keeps you off jury duty rolls; 3) common sense is that your one vote won't make much difference; 4) poor candidate info means people avoid the folly of a vote for an unknown; 5) DEMOCRATIC PARTY DOMINANCE means most feel it doesn't matter, and minority party opponents of quality are discouraged from running. 

Don't forget, mail-in voting allows many private opportunities for group and individual peer pressure. Quasi-coercive group voting is already a staple of union tactics.  Other one-on-one pressure tactics are even harder to police. No 'poll watcher' can see what's going on. That's why existing vote-influencer groups actually like all-mail balloting. Careful!