We add a paragraph to birkat hamzon for Purim (and for Chanukka). Why is there no phrase added to the me'ein shalosh (the bracha acharona) for Purim (and Chanukka)?
This question has to do with the dispute over "ve'al hannisim" or "al hannisim."
The Mishnah Brurah (תרעב) writes that one should be careful to recite "ve'al" whereas the Shulchan Aruch quotes a minority opinion that "some sefarim have the nusach of 'al hannisim'", including the Rambam in Sefer Ahavah.
This debate was sparked by two separate sources, one in Brachot and one in Shabbos. The one in Shabbos (21b; speaking of Chanukkah) reads
".לשנה אחרת קבעום ועשאום ימים טובים בהלל והודאה"
Rashi interprets Hallel as Hallel, but Hoda'ah as Al Hannisim. This indicates that he would say "Ve'al" because he would have added it to the Hoda'ah section in Shemoneh Esreh, which is a list: "Ve'al nissecha shebechol yom imanu...Ve'al tovoteicha..." and then "...Ve'al hanissim ve'al hapurkan."
The source in Brachot reads
".ברוך ה' יום יום- בכל יום תן לו מעין ברכות"
The Gemara is speaking of putting in some short summary of the day in our Shemoneh Esreh; which appears as a separate Bracha, Ya'aleh Ve'yavo.
People like the Rambam would hold that "Al Hannisim" is the summary of the day, and would put it as a separate Bracha to fulfill Baruch Hashem Yom Yom.
Your question is one of the נפקא מינות of this debate.
Rav Lichtenstein explained: Birchat Hamazon is the lengthy version of the Bracha Me'ein Shalosh. In Birchat Hamazon, we have the Bracha of Nodeh Lcha. However, in the shortened version, there is no Nodeh Lcha-like sentence. However, on Shabbos and Yom Tov, we still add in a short summary of Ya'aleh Ve'yavo in the Bracha Me'ein Shalosh. On Chanukah and Purim we do not. The reason why is that "Al Hanisim" is a bracha of Hoda'ah, not of the Yom, and so is not placed in the Bracha Me'ein Shalosh, where there is no sentence about Hoda'ah.
Hope this helps. א פרילייכן פורים everyone!