Because the ground has moved. Transition creates a gap between the situation you are in and the habits you have relied on to succeed. What got you here was credibility in your domain, delivery, and leading a team with clear outputs. Senior roles ask for something different: reading and managing the system, the room, and yourself, often simultaneously, with less certainty and more visibility.
At this level it is rarely about what you know. It is about how you show up and apply your experience to what the situation requires. The instincts that served you well before can become the thing that holds you back. The room you think you are in and the room you are actually in are not always the same. Feeling out of your depth is a reasonable response to a genuine shift in what the role requires. The problem is not capability. It is that the old autopilot responses no longer fit the new situation, and that gap does not close by itself.