The Stolen Throne of Shushan | Megillas Esther Explained (1:2)
From the book Megillas Esther Explained (Mosaica Press)
Source: From the commentary on Esther (1:2) in my soon to be published book entitled Megillas Esther Explained by Yisroel Dov Levene (Mosaica Press).

(ב) בַּיָּמִ֖ים הָהֵ֑ם כְּשֶׁ֣בֶת ׀ הַמֶּ֣לֶךְ אֲחַשְׁוֵר֗וֹשׁ עַ֚ל כִּסֵּ֣א מַלְכוּת֔וֹ אֲשֶׁ֖ר בְּשׁוּשַׁ֥ן הַבִּירָֽה׃

(2) In those days, when King Ahasuerus occupied the royal throne in the fortress Shushan,

All these happenings took place In those days, when King Ahasuerus occupied the royal throne. What was special about those days that then this happened?
It says in Koheles:

(ט)אֹהֵ֥ב כֶּ֙סֶף֙ לֹא־יִשְׂבַּ֣ע כֶּ֔סֶף וּמִֽי־אֹהֵ֥ב בֶּהָמ֖וֹן לֹ֣א תְבוּאָ֑ה גַּם־זֶ֖ה הָֽבֶל׃

(9)A lover of money is never satisfied with money, nor a lover of wealth, with income. That too is futile.

He will always continue the pursuit for more money.
Achashverosh had money and wanted to invest it into something that would provide dividends. He did this in the form of seeking to get more honor and fame by building a duplicate to King Shlomo’s throne.

(2) This King Solomon was it who had caused a glorious royal throne to be made, covered with gold from Ophir, overlaid with beryl stones, brilliants, marble, samaragel, carbuncle, diamonds, pearls, and other precious stones. For no king was the throne made like it, and no kingdom could produce similar ones. This throne was made as follows: Twelve lions of gold stood upon it, and opposite to them twelve eagles of gold, a lion opposite an eagle, and an eagle opposite a lion. The right paw of the golden lion was toward the left wing of the golden eagle, and the left wing of the golden eagle was toward the right paw of the golden lion. The sum of all the lions upon it was seventy-two, and there were the same number of eagles. Towards the top, where the king's seat was, the throne was round. It had six steps of gold, as it is written: "The king made a throne of ivory, and this throne had six steps." Upon the first step lay a golden ox, and opposite to it a golden lion; upon the second step lay a golden bear, and opposite to it a golden lamb; upon the third step lay a golden panther, and opposite to it a golden owl; upon the fourth step lay a golden eagle, and opposite to it a golden peacock; upon the fifth step lay a golden cat, and opposite to it a golden hen; and upon the sixth step lay a golden hawk, and opposite to it a golden dove. Upon the throne stood likewise a golden dove, holding a golden hawk in its claws.

The reason why he only made a replica and did not try and get the real throne is because Paro had tried doing so and got kicked by the lion and became lame, earning him his name נכה פרעה, Paro the lame.
Thus, it was בַּיָּמִים הָהֵם in those days when the throne had been completed, and he was finally able to sit on it. The possuk continues אֲשֶׁר בְּשׁוּשַׁן הַבִּירָה which was in Shushan the capital.
It started being built in שׁוּשַׁן הַבִּירָה, Shushan the capital, (either because that it was where the materials were available or where the workmen resided) and was meant upon completion to be moved to the long established
capital in Bovel.
However, when the throne could not be moved, he wanted the throne so badly that he was willing to move the years old capital to שׁוּשַׁן הַבִּירָה, Shushan the capital, all in order to be able to show off sitting on this impressive throne!
Alternatively, feeling on a high, the king wanted to change place of his capital voluntary with the idea of שינוי מקום שינוי מזל, the change in place can change the Mazal, for making even better his current successes.
If you enjoyed this, there is the whole passuk by passuk commentary on Megillas Esther, soon to be published entitled Megillas Esther Explained (Mosaica Press).