Because they were not generally to go out to war, which was the thing principally eyed in this muster, Numbers 1:3,20,45 but were to attend upon the service or the tabernacle, and therefore are reserved to another distinct muster, Numbers 3:15, Numbers 4:2, &c. And lost this should bc thought to, be designed and done through Moses's ambition to give his own tribe the pre-eminence, he assures them it was done by God's express command.
Related Commentaries of Numbers 1:49
Numbers 1:49
49 Only thou shalt not number the tribe of Levi, neither take the sum of them among the children of Israel: