For those outside of Israel the requirement is for two days of yom tov (and sometimes Rosh Chodesh), originally due to uncertainty and now due to tradition. But we don't do this for all holidays even though they involve time-bound mitzvot:
Yom Kippur: it would be impractical to observe for two days; is that the reason or is it coincidence?
Chanukah: is the 25th of the month late enough that we are certain of the date?
Purim: I have no guess here
Is there a unifying reason for when we do and don't add the second day, and if so what is it? Or is it on case-by-case basis, and if so what are the reasons for these three cases?