They had a partner airline - it no longer exists. That's really the long and short of it.

Extra long version: WN never physically operated to Hawaii, rather used the partner airline. That airline went under - in part due to running flights to Hawaii which weren't profitable. So, WN has no infrastructure in Hawaii to begin service and would have to start from the ground up, thus incurring those costs, in addition to scheduling it into the routing. It's not like WN stopped going to Hawaii - they never did, and at this time do not apparently think they should.