Question

Why do particles of collidal solutions do not settle down while in the case of suspensions they do ?

Answer 1

Particles in colloid is very small compare to that of the suspension. This makes the colloids more stable than suspension. 
Because of the larger size of particles they settle down when suspension is left undisturbed. While in the case of colloid they do not settle as the particles are very small.

Answer 2

In a colloidal solution the colloidal particles are small and they are being continuously hit upon by the molecules of depression medium.

It is very unlikely that the dispersed face particles will be hit upon by an equal force from all the sides.

Therefore there is a resultant force acting upon the colloidal particles in a particular direction.

And colloidal particles execute zigzag motion called Brownian movement.

Because of this the colloidal particles do not settle down under the influence of gravity.