Good question. I found some sources that might help you out:
it appears it was likely a combination of factors (speculation that Russell might quit basketball based on his statements, his social writings/comments, Robertson allegedly having his 'finest year in the league [p]laying a superb all around game). From a quick glance, it looks like the common sentiment is that he was snubbed for All-Star MVP voting (though unlike the season MVP which was voted for by his peers in the other players, I *believe* the All-Star MVP was decided via a straw poll by media back then), partially due possibly to his outspoken nature, and the near-boycott of the game in hopes of getting the player pensions plan pushed through (it seems Russ was seen as a ringleader/instigator).
I could be completely off base though, and it could've just been an issue of voter fatigue on the part of the players.