the goa'uld have been on earth for 10 thousand yearsand at the time of the goaulds arrival ra ruled earth but for the next 5 thousand years between when ra first came and the rebelion happened many more goa'uld ruled the planet. so when ra found out that the people of earth had rebeled under which ever goauld they were ruled by ra stoped reading and writting to stop the abadonians from finding out, which is why ra didn't take back earth, plus during this time earth had lost it's value, goauld no longer needed to take slave from earth because they already had them, earth has no naquada or trinium and earth probaly had no teritorial value any more.