Or HaChaim on Numbers 5:2/אור החיים על במדבר ה׳:ב׳

1 צו את בני ישראל וישלחו מן המחנה, "Command the Israelites to expel from the camp, etc." The reason the Torah wrote this paragraph immediately after the appointment of the Levites to their respective tasks was because G'd had commanded the Levites themselves to keep away from areas possessing a certain degree of sanctity, areas reserved for priests alone. This is why the Torah added that also the Israelites were forbidden to enter areas normally available to them if they had become afflicted with ritual impurity, for instance.