The "Al Hamichya" Inconsistency – Chanuka & Purim
On Chanuka and Purim there is a special prayer, known as the "al hanissim," which is added to the Shemoneh Esrei and Birkat Hamazon in honor of the holiday. In contrast, however, no mention of Chanuka or Purim is made when one recites the "al hamichya" blessing following a snack of mezonot foods, such as cake, cookies, hamentaschen or sufganiyot. This is somewhat odd considering that on all other holidays on which there is a special insertion added to both the Shemoneh Esrei and Birkat Hamazon, there is an insertion of some sort added to the "al hamichya," as well. This inconsistency has been noted and addressed by several halachic authorities.
One of the explanations for this inconsistency is that the "al hanissim" is a prayer of thanksgiving, which is why it is inserted to the "modim" blessing of the Shemoneh Esrei, as it too is a prayer of thanksgiving in its own right. Similarly, in the Birkat Hamazon, "al hanissim" is inserted in the second section ("nodeh lecha"), which includes a number of Divine gifts and miracles which we are thankful for. The "al hamichya" blessing, however, simply does not include a component or section which specifically discusses thanksgiving. As such, the "al hanissim" might just be a prayer which is essentially incompatible with the content or purpose of "al hamichya" and is therefore not included with it. Related to this is the idea that "al hanissim" is not an independent prayer as most other seasonal insertions are deemed to be, but rather, it is a season-specific continuation of the existing prayer in which it is inserted.
Nevertheless, these explanations are weak on several accounts. For example, one who forgot to insert "al hanissim" in the proper place in the Birkat Hamazon can still make-up his omission by reciting it in the concluding "harachaman" section of the Birkat Hamazon. This seems to imply that "al hanissim" is not entirely bound to any specific section of the Birkat Hamazon. Furthermore, there is indeed some expression of thanksgiving in the "al hamichya", albeit on a lesser scale than that which is found in the Birkat Hamazon, which should allow for an "al hanissim" insertion of some sort in the course of the prayer.
It might just be that there is simply no authoritative or bulletproof explanation for the exclusion of "al hanissim" or some other token reference to Chanuka and Purim in the "al hamichya". It is noted that one who unintentionally omitted the "al hanissim" in either the Shemoneh Esrei or the Birkat Hamazon is not required to repeat the prayer. So too, there is no compensational blessing to be recited for one who omitted "al hanissim" in the Birkat Hamazon as there are on other occasions when the required insertion was omitted, such as on Shabbat and Yom Tov. This seems to indicate that the "al hanissim" is on a much lower level of importance than the other seasonal additions for which one is required to repeat the Shemoneh Esrei should one have omitted them. It is also noted that the insertions which are customarily added to the "al hamichya" are all in honor of holidays which are of Torah origin, whereas Chanuka and Purim are of rabbinic origin.
It is interesting to note that there indeed exists an opinion that Chanuka and Purim should be explicitly mentioned in the "al hamichya", though the halacha is not like this view.  One who accidentally mentioned Chanuka or Purim in the "al hamichya" blessing has discharged his obligation and is not required to repeat the prayer due to the erroneous insertion.
 OC 208:12.
 Levush, OC 208:12.
 Levush, OC 208:12.
 Tzitz Eliezer 9:33. For much more on this issue see Rivevot Ephraim 2:185:4, 6:359.
 OC 187.
 Mishna Berura 695:15; Aruch Hashulchan, OC 208:7. But see Maharshal 48 and Magen Avraham, OC 695:9 for a dissenting opinion.
 Machatzit Hashekel, OC 208:11.
 Kaf Hachaim, OC 682:3.
 Rivevot Ephraim 8:272:1; Yabia Omer 3:36.