Why is Callisto undifferentiated?

The Galileo-determined moment-of-inertia factor for Callisto of 0.406 ± 0.039 (1σ) is consistent with only a small amount of ice–rock differentiation (<10%), because the moment-of-inertia for a totally undifferentiated Callisto is 0.38 (not 0.40). read more

Due to the lack of tidal heating and small size, callisto cooled rapidly and did not have time to completely form differentiated layers. This page may be out of date. Save your draft before refreshing this page. read more

From what I have read, Callisto does not have any tectonic activity, which makes the surface as old as 4.5 Ba old. Now, what that means is that the regolith of Callisto has not been generating/ destroyed for a long period of time, which cannot be said about the other satellites of Jupiter (correct me if I am wrong, please). read more