Why do the Testaments of the Twelve Patriarchs rank Levi the priest above Judah the king?

In these testaments each dying patriarch tells his sons to honour two brothers, Levi and Judah, and Levi is named first. Reuben says Levi will know the law of the Lord and give ordinances for judgment and sacrifice for all Israel, and that his sons should approach Levi humbly to receive a blessing. Simeon says his descendants will not prevail against Levi and that salvation will arise from both tribes. Levi's own testament explains where his standing comes from. In a vision the Most High tells him he has been given the blessings of the priesthood, and in a second vision seven men in white clothe and anoint him as priest for ever. Judah, who is promised the kingdom by Jacob, Abraham and Isaac, accepts this order himself. He says the Lord gave him the things on the earth and gave Levi the things in the heavens, and that the priesthood is higher than the kingdom as heaven is higher than earth. When he lists the patriarchs who will rise to life, he puts Levi first and himself second.