Looks weird but since windsurfing and very small boards appeared in the 90s, windsurfing moved to more windy spots, like Hookipa and then all attention and marketing was focused there. I think 15-20 knot wind spots are great too and i was once in Diammond head and was really good place....
If you say there is no one there now...then its time to plan a trip there, for sure, I dont like places with tons of people.
The question is...why none in Oahu see the chance to promote the place, rent boards, sails, and all that? Maybe there is peole that dont see the opportunity.