When Nash won his first MVP, it was basically a toss up between him and Shaq. Nash won it because he was basically the only major addition to Phoenix, yet they went from missing the playoffs to best record in the league. Also, he averaged more assists than any player had in years.
His second MVP, he put up even better stats than he did the previous year, while losing their best scorer in Amare for the season, and the team didn't skip a beat. Kobe probably was his biggest competition averaging 35ppg, but they were a 7 seed. Wade and Lebron were only hovering around 50 wins (even though Miami won the chip that year).
Nash was simply a beneficiary of circumstances.