Although Jin Zhicheng had exposed most of the spies, a few remained hidden and continued to report the situation here to Qian Sihai.
After receiving the news, Qian Sihai gathered the leaders of various major forces to discuss the matter. They ultimately decided to attack Wang Bin.
The reason was simple: Luo Feng was still alive, and as a King-level Superhuman, they didn't want to live in constant fear of his revenge. So, they decided to root out the problem once and for all.
As for the 300,000 people following Wang Bin, as Qian Sihai mentioned, their loyalty wasn't strong. Faced with their army, many would likely flee.