Those who work at setting priorities know that it's not a single-person game. You might have a good idea of what you need to get done first, but that doesn't mean other stakeholders agree with you. This article from PM Hut is empathetic to the project manager's plight. There are simply dozens of considerations when priorities are being established that conflict with each other, and knowing how to navigate those conflicts are what separate a successful priority establishment and an absolute quagmire of meetings, disagreements, and confusions:
It is often only the project manager who realizes that priorities are in conflict. For example, one department involved in a project may be incentivized to complete its activities rapidly ahead of schedule without regard for quality. Another department involved in the project may actually experience delays in completing its assigned tasks due to inferior handoffs from the other department. In this type of situation it is the project manager who must educate project participants in the realities of not sacrificing the project for the benefit of a single component.
Even if an agreement is reached, that doesn't mean that the priorities can't shift while in-flight. The project manager has to first recognize how these changes in priority impact other projects, but also use sound judgment in whether the changes are just part of the natural conflicting opinions of the stakeholders or if the changes are actually necessary for business success.