R' Hirsch in Horeb, ch. 100 para. 638 (p. 487 in the Dayan Grunfeld translation), writes regarding Rosh Chodesh and Chol Hamo'ed:
These are days which have their special significance in the Musaf sacrifice, but which are otherwise characterized by the usual activities of life. In the Musaf sacrifice, the life-concept of the people, which finds its usual expression in the תמיד, is brought into a special relationship with God. These days, which are specifically characterized only by Avodah, are ראש חודש and חול המועד. This merging of national life in God is also included in the Avodah of the Shemoneh Esreh, which introduces individual everyday normal life (יעלה ויבא)...
Paragraph 639, regarding Yom Tov:
Yet there are days which... are raised above the days of the year and should not allow life to spin along its usual course... The Divine Service, which imbues these days with their decisive significance, is definitely changed. Indeed, their task being mainly the revivifying of ideas, it is a task which must find its partial realization in tefillah.
Accordingly (as expressed in paragraph 648), Yaaleh Veyavo is placed in the location which reflects the nature of the day as expressed through the prayer of that day. On Chol Hamo'ed, which is a day characterized through the change to the Avodah of the day, has its Yaaleh Veyavo in the blessing of Avodah. Yom Tov, on which the entire prayer is changed as a reflection of the entirely altered nature of the day, has its Yaaleh Veyavo in the central body of the prayer which reflects that nature.