What might explain the divergence and evolution of these two elephant species?
In Africa, western/central elephants are smaller in size, have straight tusks and smaller ears, while Savanna elephants from the east are larger in size, have curvier tusks and bigger ears.
Any time you have a clear divergence, or separation of two separate sets of characteristics, it is because there is some importance accorded to the possession of those characteristics that enable that particular organism the ability to survive. Physical attributes can be cultured or lost based on the necessity of such a body part's importance in the organism's day to day living. Ears on an elephant are not only used to hear, they are also heat distribution mechanisms. Maybe the Savannah elephants had a greater need to distribute body heat than their western counterparts. As far as body size, maybe smaller-bodied elephants outlived their larger-bodied cousins, so the geneaology for small body size endured. For whatever reason, however, there was a set of extenuating circumstances that favored that particular animal for that particular environmental setting.