You’ve asked a very interesting question! Here’s the story.
In Jewish tradition the calendar is determined by eyewitness sightings of the full moon, and then declared by the Jewish Supreme Court, the Sanhedrin in Jerusalem. In the days of the Temple, people outside of Israel simply would not know if the previous month was twenty nine or thirty days, and therefore would be in doubt as to which is the correct night (for example) of the Passover Seder and other festivals as well. Since it generally took more than two weeks to get to the Diaspora from Jerusalem, messengers would not arrive in time, and the Diaspora Jews would have to keep two Seders out of doubt.
Nowadays, we calculate the months based on a fixed calendar. But, nevertheless, the Talmud decreed that outside of Israel the Festivals should be kept as they were in ancient times as two days, as though there was a doubt. So today in Israel the community has only one day of Yom Tov (and therefore only one Passover Seder), whereas outside of Israel they have two. The reason for the decree is protect the integrity of the observance of the Festivals. So, just in case calculations and communications collapse for some reason, people will be "safely" observing them properly. In addition, this ensures that the Diaspora maintains a feeling of connection and dependence on the "word of God, coming from Zion and Jerusalem".
The most obvious way of removing the problem altogether is if we, the Jewish People, collectively bring about the coming of the Mashiach. At that point all of the Jewish communities will come to live in Israel and we will all keep only one day as is written in the Torah. May it happen speedily in our days!
