A. Back in the days when our calendar was determined by the testimony of witnesses on the sighting of a New Moon, messengers had to be sent to all Jewish communities so that they would know when the new month began, and when the 15th (first day of Passover) would fall. Within the borders of Israel, the message would reach before the Holiday, but outside of Israel, it would take longer (before IM, faxes and e-mail) and at times the communities would be in doubt – and would keep an extra day of the Holiday – just to be sure.
This, by the way, explains why even in Israel, Jews keep 2 days of Rosh Hashona. Being that Rosh Hashona is the 1st day of the month, there was no time to inform the communities even within Israel, so they kept 2 days – to be sure.
This became the established long-standing custom, so even when Hillel (the 2nd) established the Jewish calendar (based on the Sinaitic tradition as to the precise calculation of the moon’s cycle of renewal) the communities in the Diaspora still kept their extra day. Hillel II established the Jewish calendar 1,640 years ago, and it is still in use today.
So why keep an extra day, once they knew with certainty when the Holiday was? It may have to do with – once it has been observed over time, it has become sanctified. We don’t do away with that which has become holy. And the Diaspora could use an extra dose of holiness and festival – which may be another reason to hold on to the 8th day of Passover. Either way, the extra day of holiday in the Diaspora continued to be observed even when the sages established the fixed calendar.