Two reasons.
1) Only 7 pokemon are viable defenders with the cp ranking system. For every other pokemon only attacking movesets are relevant.
2) Given the fact that few movesets will ever deter anyone from taking down a gym, even on the big 7, their defensive movesets aren't really THAT important. High cp is more important, because what good does a great moveset do you if you can't place your mon in a gym.
...and a third:
3) Even with the big 7, if you power up one with high IV's and a great attacking moveset he has a dual purpose. You can attack with him effectively or you can put him in a gym due to high cp. With a strong defensive moveset, he is really only useful as a defender.
Of course, this may change with the gym rework, who knows.