Most flights to Hawaii depart the West Coast, and in some cases it is an even shorter route than a transcon (e.g., SFO-HNL is a few hundred miles less than SFO-EWR).

Until recently United did not have so many 757s to Hawaii, though the aircraft were still old. And widebody service to Hawaii still exists, including on Hawaiian and American. Hawaiian's service is actually quite good.

As others have mentioned, it's not a very profitable route since it's filled with leisure travelers. And when I can still expect to fly an A319 between Newark and Seattle on United, I consider a 757 an improvement with the larger first class cabin and better upgrade odds.