1. The reasons behind the Muktzah prohibition:[1]

The Sages prohibited moving certain items on Shabbos in the way they are regularly moved during the weekday.[2] The following are the reasons recorded:

1. The First reason- In order that one rest:[3] For what reason did [the Sages] establish this prohibition? [It is because] they said [to themselves]: If the prophets [even] warned and commanded that one’s [form of] walk on Shabbos should not be like ones [form of] walk on the weekdays[4], and that one’s [content of] talk on Shabbos should not be like ones [content of] talk on the weekdays, as it says [in the verse written by the prophets] and “you should not talk of mundane things”[5], how much more so should ones moving [of items] on Shabbos not be like ones moving [of items] on a weekday, in order so that Shabbos not appear like a weekday in one’s eyes. [As if there were to be no differentiation in ones moving habits then] one will come to lift up and fix vessels from corner to corner, or from house to house, or to put away and carry stones and the like, being that [on Shabbos] one does not do any work and thus sits in his house and searches for something [permissible] to occupy himself with. [Thus, if he were allowed to move around all these items without restriction, he would do so in order to fill up his time] and it ends up that he did not rest at all, and has thus nullified the reason [behind Shabbos] that is stated in the Torah ‘So that you should rest’ [and thus it appears just like a weekday].
2. The second reason-In order to prevent carrying:[6] Furthermore, [another reason behind] the prohibition is [in order] to make a precaution against [transgressing the prohibition of] carrying an item outside. [As if moving objects were to be allowed freely then] perhaps one may come to forget and will carry the item that is in his hand into a public domain. [See next Halacha regarding when the decree of Muktzah due to this reason came into effect and why!] Now, according to this reason it would have been proper to prohibit even the moving of vessels that have a need on Shabbos, however [nevertheless, this was not done as] we do not enact decrees which are too burdensome upon the public.
3. The third reason-In order so the rest of the unemployed be apparent:[7] Furthermore, [another reason for the Muktzah restrictions is] because some people are [permanently] unemployed, and are thus unoccupied all their lives, such as the tourists and those that sit on the street corners, which throughout all their lives are unemployed, and thus if it were permitted [on Shabbos] to walk and speak and move items as is done on all the other days [i.e. the weekdays], then it ends up that their rest on Shabbos was not recognizably [different then their rest on the weekdays]. Therefore [since] resting from doing the above [walking, talking and moving objects] is a type of rest that is applicable to all people [as even the unemployed talk and walk and move items on the weekdays], therefore because of this the [Sages] made restrictions on moving items [on Shabbos] and forbade to move items on Shabbos with exception to vessels which one needs as will be explained.[8]

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[1] Admur 308:1; Rambam 24:12; Beis Yosef 308; M”B 308 Hakdama

^[2] However to move them in an irregular way is permitted by all objects, as will be explained.

[3] Rambam 24:12; Beis Yosef 308; M”B 308 Hakdama; This reason is not mentioned in the Gemara and was innovated by the Rambam. [Maggid Mishneh ibid]

[4] As explained in chapter 302 that one may not run on Shabbos.

[5] As explained in chapters 306-307

[6] Raavad on Rambam 24:12; Shabbos 124b; Beis Yosef 308; M”B 308 Hakdama

Why does the Rambam omit this reason: This reason is based on the Gemara ibid and it is hence most puzzling that the Rambam omits it. Some explain that the Rambam wanted to bring only his new innovated reasons. [Maggid Mishneh ibid] Others explain that the decree of Nechemia, which is the reason mentioned in the Gemara by the Raavad, only applied to a specific type of Muktzah, and this type of Muktzah is no longer relevan, as Nechmia later retracted his decree. Hence there is no reason for the Rambam to bring it if its decree is no longer applicable. [Aruch Hashulchan 308:5]

[7] Rambam 24:13; Beis Yosef 308; M”B 308 Hakdama

[8] Difference between the first and third reason: To summarize the difference between the 1^st and 3^rd reason: The first reason emphasizes the importance of making a  difference in all one’s actions done on Shabbos in order so it be clear the sanctity of Shabbos in all one’s ways. Hence they decreed against Muktzah which is one of the main proponents of making one’s Shabbos be different than the week. The 3^rd reason however does not emphasize the need of their being a difference in all of one’s ways, and rather that as so long as in one’s general activity there is a difference between Shabbos and weekday there is no need to decree against Muktzah. Thus the working man which does not work on Shabbos does not require an additional decree of Muktzah to emphasize their rest on Shabbos. However the unemployed require the decree of Muktzah in order to emphasize their rest of Shabbos.

[9] Shabbos 24:13; Beis Yosef 308; M”B 308 Hakdama

[10] The reason: As possibly this is included in the reason that lest one forget and carry the object.

[11] Igeres Hakodesh of Admur p. 151