why does a moving charge experience a force in a magnetic field?

It is the basic feature of magnetism is that, in the presence of a magnetic field, a moving charge will experience a force.

As 

F = qvB

If v is zero, force is zero.

This force on the charged particle is always perpendicular to the direction it is moving. Thus magnetic forces cause charged particles to change their direction of motion, but they do not change the speed of the particle.