Honolulu is relatively conservative compared to the rest of Hawaii because (a) There are actual rich people who vote there (all year round), (b) The whites in Honolulu are more representative of whites on the mainland, while the whites elsewhere in Hawaii vote like whites in resort areas, (c) military installations, (d) There are more native Hawaiians in the rural areas than in Honolulu, and (e) Ethnic identity and tensions are higher in Honolulu than elsewhere in the state, prompting some level of ethnic voting that allows for ethnicity-based political machines (for both parties, but that tends to be a moderating effect on the overall vote)
The highest concentrations of native Hawai'ians are in the non-urban parts of O'ahu. I'm too lazy to look up detailed stats, but Honolulu County's share of both Pacific Islanders and people of two or more races (a lot of whom are part Native Hawaiian) are only minimally below the state average... and I'm pretty sure I remember there's a sizable Samoan community on either Maui or Hawai'i Island.
Thing is, outer O'ahu's natives are mostly Mormons, and are definitely one of the more Republican voting blocs in the state (though not monolitihically Republican or anything like that.) Of the two precincts McCain won in the state outside of Ni'ihau, one's in the heart of the Native Hawai'ian Mormon country and one's in the heart of the Military-Industrial Complex around Pearl Harbour.
The richest whitest bits of Honolulu proper do tend to vote Democratic by smaller margins than the state average as well, so there's some truth to that, but "Military+Mormons" (with Mormons just added as an afterthought) is a better explanation. Really it basically comes down to the military presence.
