Question

Why is always a changing magnetic field required to produce electric current in the circuit ?

Answer 1

You can't get something out of nothing, from faraday-lenz law the rate of change of magnetic flux is the emf produced to oppose the change in flux\[E=-\frac{d \Phi_B}{dt}\]
A magnetic field will attract or repel the electrons in a conductor in some way, by changing the magnetic field, it will cause the electrons in the conductor to tend to move in accordance to the change in attraction of the magnetic field. This produces a current.

Answer 2

Thanks