Why does air pressure decrease from the troposphere to the exosphere

The correct answer is - because of lower density.

The air pressure is practically representing the mass of the air. The denser the air is, the heavier it will, thus it will create more pressure, and the less dense the air is, the lighter it will be, thus creating less pressure.

The Earth has several atmospheric layers, with the troposphere being the lowest, and the exosphere being the highest. The troposphere and the exosphere have a significant difference in the air pressure though. The reason for this is that the troposphere has much more gasses in it, both in number and in quantity resulting in denser air mass, thus having higher air pressure. The exosphere, on the other hand, is very poor in gasses, having only few, and their molecules are very dispersed in the space, resulting in much less density, thus much lower air pressure.