I think it stems from the lack of legit superstars.
Think about it. Today's superstar cons utes the likes of McGrady, Pierce, Ming, Carter, etc...
None of them are complete. Not of them are that great. None of them ever have much playoff success. None of them can individually lead their teams.
Obviously there's players like Duncan, Garnett, Shaq, Wade, LeBron, Kobe, Dirk who all could/can, but Duncan and Shaq breaking down (please take with a grain of salt), missing time and becoming just great players. Garnett and Kobe had each taken a step back in team success, despite great individual success. Wade and LeBron are still incomplete players and struggle against the best teams (LeBron more than Wade, but Wade averged around 6+ TO's per game without Shaq in the lineup) and Dirk has a stigma surrounding him.
So Nash sneaks in, mainly because his teams always seem to do very well, no matter what. They exceed expectations, break, set records and always compete...
He's not at their level, but when their level isn't exactly "All-Time" any longer, than they have no one to blame but themselves.
There's no line of All-timers anymore. You once had player like Magic, Bird, Thomas, Jordan, Barkley, Drexler, Pippen, Hakeem, Ewing, Robinson, Malone, Stockton...
Now we have LeBron, Dirk, Iverson, Kobe, Amare, McGrady, Wade, Duncan, Shaq, Garnett, Nash...
Just about every player is better from the 1980-1992 era than the 1993-2005 era.
This current era, save for Shaq, Duncan, Kobe and possibly James just don't add up.
Therefore, under the scope of history, you can't expect this era's MVP to always measure up.