Rosh Hashanah is celebrated as two days everywhere because it occurs on the first day of a month. Messengers were not dispatched on the holiday, so even people in Israel did not know whether a new moon had been observed, and everybody celebrated two days. The practice was also maintained after the mathematical calendar was used.
Yom Kippur is celebrated only one day everywhere, extending the holiday's restrictions for a second day would cause an undue hardship.
Chanukkah, it remembers the Temple after it was spoiled by the Greeks.