Musaf of Shabbos Rosh Chodesh

I was wondering why the final paragraph of the berocho of kedushas hayom on Shabbos Rosh Chodesh contains most of the regular text from Rosh Chodesh ("chadeish oleinu es hachodesh hazeh..."), but leaves out the regular text from Shabbos (kaddesheinu bemitzvosecho... vetaheir libbeinu... {veyismechu vecho | veyanuchu vo} Yisroel {ohavei | mekadeshei} shemecho"). This is the case in siddurim of both Minhag Ashkenaz and Minhag Polin.

The Aruch haShulchan (425:2) points out that the Sefaradim do include this on Shabbos Rosh Chodesh, and writes that he isn't aware of any good reason why it's left out of the Ashkenazi siddurim, so therefore he does say it.

Is anybody aware of the reason why this paragraph is not said according to the traditional Minhag Ashkenaz?

DCohen