That’s always intrigued me as well; I’ve been told that the answer is simply ?? ???? – once there are two days for some Yomim Tovim, we do it for all of them (except Yom Kippur).
Because in Eretz Yisroel they never had to keep two days (because they knew, through a system of fire signals, which day was the correct one for Yom Tov). The exception was Rosh Hashana, and even in Eretz Yisrael Rosh Hashana is two days.