Why did Lucas choose to keep memory of R2-D2?

This is quite popular.. C3P0 had a memory wipe, but R2-D2 never had a memory wipe.

This question is from out-of-universe perspective. Why did Lucas choose to do this after Prequel Trilogy? What did he try to make compatible with the Original Trilogy? What would have been different if R2-D2 had memory wipe and C3P0 never had memory wipe, or both had memory wipe, or memory wipe thing wasn't even mentioned?