Why was the island hopping campaign in the pacific so deadly for both sides?

Answer 1

Each of the islands fought over became a "Do or Die" struggle for the common Japanese infantryman who knew that there was no return home should the Americans persevere .
This resolute mind-set meant that the two enemies would be in a bitter struggle to kill each other .
Refer to the "Blowtorch and Corkscrew" tactics in the related link below .

Answer 2

The "Island Hopping" & "Leap-frogging" campaign by the US & Australian military in the Central & South Pacific in WW2 was designed to reduce Allied casualties while maintaining the Allied advances. It was successful because it skipped many Japanese strongholds and attacked weaker defended locations behind Japanese strongholds. The key was the Allied ability to quickly build air bases on their new locations to use airpower to isolate the Japanese from resupply. This allowed the Allies to starve Japanese garrisons & keep them contained with a minimal loss of US & Australian lives. The bloody battles occurred at those specific locations that the Allied forces invaded & seized. The Japanese were usually wiped-out. The Allied deaths were high but usually much less than the Japanese. This strategy meant that the Allies invaded or landed at only 1/4 of the total possible locations and achieved the same objectives as a "clean-sweep" strategy would have achieved, with only 1/4 the losses. The Allied objective was to limit Allied losses, not to wipe-out all Japanese.