Rashi: People who do not live in a walled city.

Rashi citing Megilah 2b: A Gezeirah Shavah "Perazi-Perazi" from the cities of Sichon and Og teaches that it depends on whether they were surrounded at the time of Yehoshua bin Nun ¹ .

Torah Temimah citing Megilah 19a: The repetition teaches that a Ben Ir for one day is called a Ben Ir. (If a walled city dweller went to an open city and will be there on the day of the 14th, he reads on the 14th, like them.) Similarly, a Ben Kerach for one day is called a Ben Kerach (if he will remain there until the morning of the 15th).

Vilna Gaon: Esther requested an extra day to kill in Shushan also on the 14th, for they feared the Sarim, who saw both letters. Therefore they made a new day that was not written initially, to show that the king commanded; they did not do so on their own. They did not fear the commoners at all, who thought that from the beginning [the edict was for the Jews to be ready to kill their enemies]. They feared only the Sarim, who live in Medinos, i.e. walled cities. All Jews in walled cities needed that they will kill in Shushan also on the 14th ¹ , so they do like Shushan. Sarim were not in open cities, so there, the primary miracle was on the 14th ² .

Malbim: The first year [of the salvation], only the Perazim accepted to make Mishteh and Simchah. Bnei Kerachim did not. This is because the content of Haman's sealed letters was not revealed. The Sarim hid them, like I explained above (refer to 9:3:1:1). No one knew that the Jews' enemies were allowed to kill them. The Jews thought (a) that the initial rumors that there was an edict to eradicate the Jews was totally false; it never came from the king, only from Haman, or (b) the edict was true, but the king totally nullified it and gave a new edict. Either way, there was no miracle on the 14th. They were not in danger, for the nations would not strike them, since the king's army in walled cities would help them. Therefore, they did not make days of Mishteh. The Perazim were in danger. Their enemies fought them, and the king's army was not there. Only they made days of Mishteh.

Malbim (21): Surely Haman's decree was not limited to the 13th. He wanted to eradicate Yisrael. If they could not finish on the 13th, surely they may kill the rest afterwards. Mordechai's letters let the Jews kill their enemies only on the 13th, for it was not a decree to eradicate their enemies. We see that Esther needed to request a new edict to let Jews kill their enemies also on the 14th in Shushan! Had the Jews' enemies risen to kill them on the 14th, the Sarim could not let Jews fight them! Miraculously, the Sarim hid the first edict and the enemies did not rise against the Jews; then, the miracle was known. They made Purim on the day that they rested; in the Kerachim, they did not rest until the 15th. There were no Sarim in the open cities. They overpowered on the 13th, and there was no danger on the 14th and they did not fear, so they made Purim on the 14th.

Torah Temimah citing Megilah 2b: We infer that walled cities celebrate the 15th ¹ .

Vilna Gaon: These four correspond to the four matters that Haman wanted to do, like I wrote above ¹ (refer to 3:13:2:1).