Something in Su Xin's words struck a chord with Su Mingqi. As Su Xin said, their collateral lineage lacked not power, but confidence.
When facing the direct lineage, they were intrinsically meant to take the backseat, a truth even Su Mingqi was not immune to.
Remember that Su Zhongying and the others were prepared to swallow their pride and keep the peace. It was only after being provoked by people like Wei Changdong that they decided to approach Su Mingqi.
At that time, Su Mingqi himself could have proactively sought out Su Zhongying, but he chose to wait for Su Zhongying to come to him, hoping to gauge the level of spirit remaining in these branch-lineage disciples.
However, while Su Mingqi was tempted, reason told him that if they openly clashed with the direct lineage, both sides would only end up hurting each other, potentially leading to the Su family's downfall.