Question

reasons
why using mirrors to generate electricity is not a common practice.  

Answer 1

mirrors are made of glass and glass isn't the greatest conductor.

Answer 2

A big part of the reason that mirrors are seldom if ever used to generate
electricity is the simple fact that there is no way to generate electricity using
mirrors. They are as useless for the purpose as smoke is, although there are
those who have used both items simultaneously to create the impression that
they have succeeded in that attempt.