That's quite simple. As one joke says: "Nowadays, the "against all" option is called KPRF".
KPRF were the only party who had the chances of constituting at least some opposition to ER in the Duma. Casting a vote for anyone else would only spread the support pretty thin, besides, LDPR are clowns and SR are just the same thing as ER (though they try very hard to look this way). Guess who'd left?
People voted not because they shared communist views but because they wanted to vote for someone who is definitely opposed to the 'common enemy'. An enemy of my enemy is my friend.