So you are ignoring the fact that they are movies from books, that have been released in order (I,II,III,IV,V,VI,VII,VIII,IX (Yes, NINE, but Lucas since didnt wanted to made the rest in movies)) and that he didnt used so much CGI before because of the same reason that he launched IV, V and VI first. Its because the technology at time couldnt do what he wanted, since the first ones needed more things to do all these giant things and droids without looking crap he waited patiently and did what he could at time, that being IV, V and VI. :p