1. Question: Why are the matzos set aside for the mitzvah of eating them on Pesach only muktzah when Pesach eve falls out on Shabbos but not on any other Shabbos?
A. Muktzah literally means set aside. It is a rabbinic restriction prohibiting the movement of certain objects on Shabbos in order to preserve the sanctity of the day and avoid possible desecration.
B. Three examples of categories of muktzah which might all apply in this case are (a) muktzah machmas chisaron kis: delicate objects used for their intended purpose because of their costliness like stationary, fountain pen, violin, electric microscope, shechita knife or camera, and (b) muktzah machmas mitzvah: Objects whose use are limited to involvement with mitzvos like a lulav and esrog, a megillah, or shofar. (c) muktzah machman gufo: Object that are not utensils and have no ordinary Shabbos function like raw potatoes, untithed fruit, dirt, rocks, money, etc.
C. It is rabbinicly forbidden to eat matzah on erev Pesach.
D. All other Shabbosim, Rav Shlomo Zalman Aurbach says the matzah is like yom tov clothing which you would only wear on yom tov, not Shabbos.
Answer: Since it can’t be eaten or even given to the birds on erev Pesach then it is muktzah on Shabbos erev Pesach but on any other Shabbos, though you would not eat them and you are saving them for Pesach but they are potentially edible so they are not muktzah and can be moved on any other Shabbos. (See back of Dirshu Mishna Berura page 7, siman 443, footnote 8)