Initially, Tang Qing had considered supporting Wei Jun to seize the power, since he had the foundation, connections and experience there, which would make for a quicker takeover. It would also have seemed more 'legitimate' with sufficient reasoning to not provoke too much resistance from surrounding forces.
However, upon careful thought, he dismissed this idea. Since these territories were acquired by his men, it made no sense to willingly hand the reins to others. Even though Wei Jun was obedient to him because of his superior strength, who could guarantee that things would stay this way in the future? No one likes to be manipulated. So, to avoid potential future conflicts and betrayals, Tang decided that it would be more prudent to let his men maintain control over the territories.
Of course, the plan would not involve killing their leader immediately and taking his place, as they were not beasts that could ascend to the leadership merely by killing the alpha.