Although disease was probably the primary factor, as other answers have addressed, there was another factor that negatively impacted the Hawaiian population. Hawaii was an important stopping point for trade vessels. It was not uncommon for ships to take on board Hawaiians as laborers in various roles. Many, of them never returned, but they left their mark in far flung places. For example, there is a county in southwest Idaho called Owhyee. It's name comes from the history of Hawaiians that accompanied the fur trading parties operating out of the northwest coast.
See Hawaiians at Fort Vancouver
In 1840, Kamehameha III, faced with the seeming threat of racial extinction due to depopulation by both emigration and disease, enacted a law that required captains of vessels desiring to board Hawaiians to obtain the written consent of the island governor and sign a $200 bond to return the Hawaiian within the specified time.
for an idea of the scale...
Hudson’s Bay Company Governor Simpson, on a visit to Hawaii in 1841, reported that
"About a thousand males in the very prime of life are estimated annually to leave the islands, some going to California, others to the Columbia, and many on long and dangerous voyages, particularly in whaling vessels, while a considerable number of them are said to be permanently lost to their country, either dying during their engagements, or settling in other parts of the world."
Governor Simpson began to worry about the number of Hawaiians employed in the Company service and in 1842 ordered McLoughlin to hire no more. McLoughlin, faced with problems of increased production, disagreed and disobeyed the explicit order and hoped the home office in London would understand the necessity and overlook this breach of discipline.