There is simply no precedent for a guy getting MVP when his value is entirely on the offensive side and he averages only 15 ppg. Yes, I know he contributed a lot with his assists and floor generalship. But other guys who did that still had to score a lot more to get MVPs.
For example, Magic Johnson got MVPs during the 3 seasons with his highest ppg output: 23.9, 22.5, 22.3. (And lets not forget that Magic, although no defensive stopper, was not nearly the liability on defense that Nash was. Also, Magic was a big-time rebounder from the backcourt.) Magic did not win MVP when he scored in the high teens (and even then, he never scored as little as 15 ppg).
John Stockon never won an MVP, and neither did Jason Kidd (both were far superior on the defensive end than Nash as well). Isiah Thomas never won an MVP, but got the most consideration during his highest scoring seasons. Chris Paul got the most consideration during his highest scoring seasons.