Because the islands of Hawai'i are located about 2,300 miles away from any continents, they are the most isolated places on the planet. That means plant and animal life arrived very infrequently and then evolved in isolation from the rest of the world. For this reason, many of Hawai'i's plants have evolved to be endemic (native to Hawai'i).
Arriving sporadically as seeds or spores attached to migrating birds or insects caught in high winds, these early plants flourished with little to no opposition in Hawai'i, and evolved to reflect this. With no grazing animals on the islands, plants in Hawai'i needed no defense mechanisms. That's why Hawai'ian holly is spineless, and Hawai'ian nettles don't sting. Hawai'ian mint is tasteless, where normally a strong taste fends off potential grazers.
Once Polynesians and later Europeans arrived on the islands, they cleared forests and introduced non-indigenous plants and animals, leading to the extinction of many endemic plants.