Why does the value of g reduce as we go deeper inside the earths crust?
You misunderstand the formula. The acceleration from Newton’s law is between two point masses and is a vector quantity. Calculating the acceleration external to a spherically symmetry body produces the same result so commonly the same formula is used to calculate orbits but when you burrow inside a body things become more complicated. Each little piece of mass still obeys Newton’s law but to get the resultant acceleration you have to sum (integrate) over the mass distribution of the Earth. In particular as you go deeper into the Earth only the mass in the spherical ball below you contribute to the acceleration. All the mass in a spherical shell above you (defined by the inner radius here you happen to be) vectorially cancel out hence g decreases. At the center g (but not pressure) is zero.
