why is it difficult to hold a school bag having a strap made of thin & strong strip?

The broad straps of school bags reduce the pressure on the shoulder or on the hand. 

As we know that

Pressure = Force/Area

So, increasing the area decreases the pressure.

In case of thin straps the, the area is small and hence pressure is large. So it is difficult to hold it.For larger straps the area is large thus reducing the pressure can be easily carried.