It's an interesting and very pertinent question! 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 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, Festivals 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 Seder, whereas outside they have two. The reason for the decree, is just in case calculations and communications collapse for some reason, people will be "safe", and mainly to maintain outside of Israel a feeling of connection and dependence on the "word of God, coming from Zion and Jerusalem".
The most obvious way of removing the problem all together is if we, the Jewish People can collectively bring about the coming of the Messiah - at that point all the of the Jewish communities will be brought to Israel and we all keep one day. May it happen speedily in our days!
