Question
Why do we make such a fuss about Purim when the whole story was a minor episode in the life of a Diaspora community, away from the mainstream of Jewish history?
Answer
This is the generally accepted interpretation of Purim, but it is quite wrong. The fact is that the real story is far 
Uncanny modern implications for both Israel and the Diaspora.
more important than most people realise; and not only is it part of what the questioner calls "the mainstream of Jewish history," but it is actually integrally connected with the land of Israel and even has uncanny modern implications for both Israel and the Diaspora.

Uncanny modern implications for both Israel and the Diaspora.
more important than most people realise; and not only is it part of what the questioner calls "the mainstream of Jewish history," but it is actually integrally connected with the land of Israel and even has uncanny modern implications for both Israel and the Diaspora. Persian political intrigue and personal pique in the palace? That's just the sideshow. Read the megillah in conjunction with the Book of Ezra, look at Rashi, and you have the real story.
The first step is to peruse Ezra chapter 4, where you read that when the Jewish exiles returned from Babylonia to rebuild the Temple, the "enemies of Judah and Benjamin" offered to help, were rebuffed and then made allegations against the Jews and sought to have the building work stopped. Especially relevant is the verse, "And in the reign of Ahasuerus, at the beginning of his reign, they wrote an accusation against the inhabitants of Judah and Jerusalem." (Ezra 4:6)
Who wrote the accusation? Rashi (on Esther 9:10), quoting the Seder Olam, says it was the ten sons of Haman. Haman, of Agagite descent, was already known as "the enemy of the Jews", i.e. of the Judeans, those who had returned to Judea and Jerusalem (Esther 3:10; 8:1; 9:10, 24) and was probably involved in the move to prevent the rebuilding of the Temple.
Mordechai, too, was not just a nice Persian rabbi; he is also mentioned in Ezra (2:2) and was a leading Jewish exile who had returned to Judea. He had been sent to plead the Judeans' case before Ahasuerus, ruler of a vast empire "from India to Ethiopia." (Esther 1:1)
What was being played out before Ahasuerus in Shushan was the battle of the leaders - Haman and Mordechai - in relation to events in Jerusalem and the future of the Holy Land. Were the Jews grateful to Mordechai for his efforts? Not all (Esther 10:3). The minority, says Rashi, believed he should not be so involved in politics, but get back to his Torah learning.
The Purim Feast
An old rhyme says, "Haman was beat, so let us eat!" Not just because eating creates a happy mood, but because our survival in ancient Persia followed the royal feast at which Esther secured the cancellation of Haman's plan to destroy the Jews. But when should we eat?
Most years, the Purim se'udah (feast) takes place late on Purim afternoon despite the normal rule that mitzvot should not be delayed. Purim seems an exception to the rule, presumably because the observances of the day include sending gifts of edible foods (mishlo'ach manot) to one's neighbours and friends, and giving charity to the poor (mattanot la'evyonim). Until these requirements have been met, enabling other people to enjoy the festivities, our own celebrations cannot in all conscience commence.
The Jewish ethos of celebration always says that one must not be selfish or put the needs of others last. This is why every community ensures that the poor have the means to celebrate Pesach and, indeed, every happy occasion in the year - especially Shabbat. Some of the greatest acts of quiet charity in Israel enable disadvantaged families to have food and wine for Shabbat.
Mordechai, too, was not just a nice Persian rabbi.


Mordechai, too, was not just a nice Persian rabbi.

This year, with Purim on a Friday, we have to make sure that the feasting does not get in the way of the observance of Shabbat. Since the se'udah can be held at any time during the day, on a Friday it is customary for it to begin earlier than usual. Some communities actually time it for the morning immediately after the megillah reading. In cities in Israel (notably Jerusalem) that were surrounded by walls at the time of Joshua, the se'udah is postponed from Friday. Extra delicacies are added to the Shabbat meals in honour of Purim and the se'udah itself is on Sunday (Shulchan Aruch, Orach Chayyim 688:6).
Haman's Admirers
Many communities that escaped from danger instituted local Purims to mark their deliverance. Books about the history of Purim often narrate these events, but they do not always explain that the anti-Semites responsible for imperilling the Jews frequently deliberately modelled themselves on Haman.
An example is a German baker, Vincenz Fettmilch, who, after announcing in 1614 that he was the new Haman, went on to foment attacks on the Jews of Worms and then Frankfort. The Jews were saved because of the intervention of the local governor, who, fearing that the riots would get out of hand, sent in his troops to suppress the attacks. Like the original Haman, Fettmilch was hanged, and the Frankfort community proclaimed the institution of Vincenz Purim.
These are some other famous local Purims:
The Gunpowder Purim
In December, 1804, there were terrible explosions at the gunpowder factories situated near the Jewish section of Vilna. Amongst those who survived were the Danzig family from whom emanated one of the great compilations of Jewish law.
The Snow Purim
This was observed by the Jews of Tunis to mark a huge snow storm in 1891 in which one of the synagogues provided food and shelter for many people. When the storm abated and people could go home, the synagogue roof collapsed, but (Baruch HaShem) no one was hurt.
The Window Purim
This emanated from Hebron in 1741. The local authorities had imposed a massive tax on the Jewish community. The Jews, unable to find the necessary money, assembled to pray. The next morning, a lad found on the synagogue window sill a bag full of money, which was used to pay the ransom and save the community.