The middle of tectonic plates tend to have fewer mountains than locations near tectonic plate boundaries. what might be one possible explanation for this?

This is because the mountains are formed by the collision between the tectonic plates.

Explanation:

The mountains are formed by the shock that occurs when the tectonic plates collide with each other. By the force of this shock, the surface of these plates folds, generating a mountain on the Earth's surface.

As these shocks between the tectonic plates have more strength and intensity in the regions of the edges, it is easier that we can see the existence of mountains in the regions that are on the edges of the tectonic plates than in the regions that are in the middle.

layer 3 is younger than layer 6.