Well you can get the answer to this question by reading fundamental laws of mechanics by I.E IRODOV. His book has given a complete discussion on reference frames in his 2nd chapter Basic equation of dynamics.
In that chapter he stated that we have a 'heliocentric' reference frame which is assumed to be inertial and is attached to the reference frame of sun.All reference frames are classified with respect to this frame.
He also gave a beautiful and a graspy definition of inertial reference frame in which he explained that a frame in which a body experiences no force due to the frame itself and all the forces on that body are due to its interaction with other bodies is called an inertial reference frame.
Much like our solar system is in a stable orbit around the sun, the vast majority of a galaxy is in a stable orbit around the black hole, with no real reason to go plunging towards the very centre of the galaxy. The book defines it in such a way to assume that heliocentric reference frame is inertial
so that we can observe nature in our solar system. But this fact is not universal.