It seems to me that the writers were running out of Gen V pokemon that were not legendaries to give to the major rivals. I could not have seen Stephan own a Liepard
, or Kotetsu owning a Hydreigon
. Again, there are only a few non-legendary pokemon that have yet to debut in Unova: Zoroark
, and Eelektrik
are the others, and two of them are not fully evolved. Most of the evolutionary families in Unova either belong to a main character, a rival, or a recurring COTD. The writers don't like featuring pokemon from the same family belonging to two different trainers unless one of them was a main character.