So what, dozens airlines fly to Hawaiian Islands .................. 
If by dozens, you mean five, then yes, dozens fly.
The five domestic carriers that fly to Hawaii are:
Alaskan airlines
Hawaiian Airlines
American Airlines
United Airlines
Delta Airlines
So why does Southwest matter? Because in every market they enter, prices drop. They're the largest low cost airline in the world. The one place they didn't fly in the US, that their customers most often requested is Hawaii. For inter island flights, there is only one airline, Hawaiian, so competition is welcome.