Because there is just a limited number of medals for all sailing sports in Olympics.
Windsurfing i guess have 2 medalls asigend, and they are given to RSX men (one) and RSX women (the other).
Supposing we want to include Serenity sailing or Freestyling or Slalom, we need to have more medals to give...and that will mean to find 2 medals of ISAF for this. Asumming the number of medals for "sailing sports" is limited, we need to find the way to take medals from other sailing spots to add them to windsurfing. I see that difficult because Olympics try to have most represantive sports in Olympics.
Yes, its possible, but also its difficult and other sailing sports can consider also to have those medals asigned to other sailing sports waiting for medals to be considered Olympic sports.