Getting WWII Japanese troops to surrender was extraordinarily difficult. They were well dug in on Okinawa, and their mission was to waste the Americans' time and munitions. Trying to bomb the Japanese troops into surrender would have made fulfilling their mission easy for them. All they would have had to do was dig deeper, and not surrender.
To clear Okinawa that way and be certain there was no resistance left, it would have been necessary to pulverise the top few metres of the entire island. This would have been very time-consuming, and would have killed all the inhabitants: Okinawa had a sizeable civilian population, and wiping them out would have been a blatant war crime. It would also have drastically reduced the value of the island as a base for the invasion of Japan.