Very good question! Answer is my own thinking, here...
The wording in the Musaf paragraph has the word כמדובר - "as it is said". This means, that the measurements are specifically said in the same place that the sacrifice of that day is mentioned in the Torah, specifically, in parshat Pinchas Bamidbar 29:1-39.
The only times the specific measurements for the wine libation are explicitly mentioned is for the weekday offering (verse 7) and Rosh Hodesh (verse 14). They are not mentioned explicitly for any other holiday, and these amounts are inferred from Bamidbar 15:3-7.
This logic raises the question - Why doesn't Rosh Hodesh mention the measurements explicitly in the Shmoneh Esreh, if it is mentioned explicitly in the Torah? Perhaps, the authors of the siddur wanted the text to match the other holidays. (I'm unaware if Rosh Hodesh is called a "mo'ed". I don't think it is.)