Why does nuclear charge not decrease down a group?

This has more to do with the locations of the electron than the number of protons in the nucleus. Look that the basic electromagnetic force formula: F = kq1q2 / r^2. Notice that the increase of distance (r) makes a bigger influence that the increase in charge q1, because the distance is squared. So because the outer-most valence electron shell is that much further away, the valence electrons are held so much less strongly, despite the increase in proton count.