A question is brought down in the sefer Poseach Shaar. If messengers were required in the months of Kislev and Adar because of Chanukah and Purim, shouldn't there be a halacha that in the faraway places, where the messengers cannot reach in due time, that there should be an extra day of Chanukah and Purim because of the doubt as to the correct day? this is indeed the reason why the other festivals have an extra day. It would emerge that there should be nine days of chanukah and two days of Purim nowadays in the Diaspora.
Abudraham writes that this is only required on the Biblical festivals but we do not have this stringency on Chanukah and Purim which is only Rabbinic.
The Mordechai states that in the Meggilah it is written "V'lo yaavor," the meggilah should not berecited on any other day but the fourteenth of Adar. Sefer Dovev Meisharim (1:15) asks on this that the verse would only answer why the meggilah is not recited on any other day but there should be an extra day for all the other obligations of Purim.
Minchas Chinuch writes that in the days that Rosh Chodesh was determined through the witnesses and Beis Din, there actually was nine days of Chanukah in the faraway places. He states further that in the future when the Beis Hamikdosh will be built, there will also be nine days. Nowadays, that we are experts in the correct days of the month, there is no necessity to institute an extra day since it is only Rabbinic.
We can answer the famous question of the Beis Yosef according to the Minchas Chinuch. He asks that there should be seven days of Chanukah and not eight because the miracle of the oil was only for seven days. We can answer that the reason there is an eighth day is because of the doubt as to when Rosh Chodesh was.