The Guarding by the Levites in the Temple Nowadays

Shalom and blessing. 

There are several reasons for this:

A. The Rambam wrote that the guarding of the Temple is for the honor of the “palace of the King,” and this does not apply to a destroyed Temple. 

B. The Rosh wrote that the obligation to guard is so that we not divert our attention from the Temple, and this does not apply when there is no Temple. 

C. In Sefer HaChinuch it is written that the reason for the guarding is to humble the hearts of those who come to serve in the Temple, and this does not apply in our times, when the service is not performed there. 

D. The Avnei Nezer explained that guarding is a form of Temple service, and one who is tamei is disqualified from service even if he performs his service outside the Temple.

E. The Avnei Nezer further explained that since today we do not know the divisions of the Levites, we must be concerned for the law of “משורר ששיער חייב מיתה” (“a singer who served as a gatekeeper is liable to death”). Therefore it is forbidden for a Levi to guard, lest he be from among the singers.

[Indeed, according to the Gra, who holds that the obligation to guard is so that those who are tamei not enter the Temple, the law of guarding applies even today.]

Rambam, Hilchot Beit HaBechirah 8:1
Rosh (beginning of Masechet Tamid). 

Sefer HaChinuch (mitzvah 388)

Avnei Nezer (Yoreh De’ah, siman 449)