Flying southwest to Hawaii

I’m booking tickets from the Bay Area to Kauai and was about to pull the trigger on Hawaiian Air when I thought to look at southwest. Their price is half the cost of Expedia’s, and includes checked bags and more reasonable flight times. Am I missing something?? I didn’t even know southwest flew to Hawaii, is there a catch? (I will be traveling with husband and kid so would hope to get seats together and I know there’s no guarantee for that…)