In many places Tanach describes sinners as being "drunk". And nowhere is this description more appropriate than in describing Jews who do not make aliyah.
One symptom of the drunkenness of Jews still in exile is a severe case of double vision. Whenever they look at the calendar, they see each holiday twice. Two days of Shavuot, two seders on Pesach, and so on. If they could see clearly, they'd know that each of these holidays takes place on one specific day. But due to their double vision they celebrate each holiday twice, on the correct day and also one day later.
The only holiday when this does not happen is Yom Kippur. Then - because they are fasting and can't drink - they celebrate one day like they are supposed to.
There is also one day on which the pattern is reversed ("venahafoch hu"). On Purim, Jews in exile celebrate one day. But Jews in Israel celebrate two days: Purim and Shushan Purim. The one time all year when you are required to be drunk and have halachic "double vision", Jews in exile are unable to. Alcoholics cannot simply enjoy a single drink like the rest of us. They have to abstain and miss the occasion, lest one drink become two or three or four drinks and they fall back into the cycle of their addiction. So too with Jews outside Israel. They cannot celebrate Purim for two days, like it is intended to be. Instead, they must make do with the relative sobriety of a one-day Purim.
Luckily for them, it's not too late to cure their "alcoholism". I hear Nefesh Benefesh is starting a new 12 step program...