The Levites had led Israel into serious sin. The people turned away from the Lord and worshiped idols.
Because of this sin, the Levites could no longer enter the inner court of the temple as the priests did. This was one result of their disobedience. However, God still showed them grace. They would continue to serve by helping the people and by killing the animals offered as sacrifices.
The people as a whole were now kept at a greater distance from God because of their idolatry. Before the exile in Babylon, they killed their own sacrifices(see Leviticus 1:5, 11). Now they had to give the animals to the Levites, who would kill them on their behalf.