If one eats enough bread to be satisfied, the chiyuv of ברכת המזון ("the beracha acharona for bread") is d'oraysa. According to many opinions, this is not the case for other berachos. Some may dispute this for "ברכה אחת מעין שלוש" ("al hamichya"), but anyways eating that much mezonos (if its פת הבא בכיסנין ) will normally require a full ברכת המזון .
The Torah states:
וְאָכַלְתָּ וְשָׂבָעְתָּ וּבֵרַכְתָּ אֶת ה' אֱלֹהֶיךָ עַל הָאָרֶץ
הַטֹּבָה אֲשֶׁר נָתַן לָךְ
Part of the Chiyuv of Birkas haMazon is to thank God for the food, the land and to thank/pray for Jerusalem. While one could perhaps fulfill the d'oraysa with "ברכה אחת מעין שלוש", full berachos were created for each part due to its higher-level obligation.
In short, since its a d'oraysa, there's a longer blessing.
(Not sure how to answer for those who hold "ברכה אחת מעין שלוש" is also d'oraysa.)