A simpler one is that: aliens are exotic creatures with accents. Humans are not. In the OT, we mostly see humans, while in the PT we see many aliens. Hence the PT has lots of accents. All the aliens in the OT--Yoda, Bib Fortuna, Greedo, Jabba, etc--have accents of some kind, they just arent plentiful in number as in the PT. Now, how to account for the fact that the Empire developed its own internal British accent is another thing...