All Yomim Tovim we celebrate in Chutz Laretz an extra day because of A safek, why do we not do the same for Chanukah?
Minchas Chinuch argues that indeed, when the Sanhedrin was functioning and we used an observation-based calendar, Chanukah in outlying places would have had to have been celebrated for nine days. "When the Beis Hamikdash is rebuilt, speedily in our days," he says, "and we go back to sanctifying the months based on observation - then faraway places (for Eretz Yisrael will spread out to include all other lands) will surely have to keep Chanukah for nine days."
We only add an extra day for a D’oraisa like the ‘Sholosh Regalim’. Chanukah is a D’Rabbanan. (See Taamei Minhagim 864 in the name of the Avudraham)
In addition as Chanuka starts on the 25th day of the Hebrew month we can presume that the Sheluchei Beis Din would have arrived and everyone would know when it actually began.
I just heard another (Chassidic) answer in the Name of the Sfas Emes:
Being that Chanuka and Purim happened relatively recently we relate to them better. Hence we are able to achieve the spiritual boost from these Holidays in a shorter time relative to the De'orasia (Biblical) holidays which happened in the more distant past, to which we are less connected.