Comment by DiogenesKynikos

Two things:

1. The media would ignore him if he did. He wouldn't get to take part in any of the debates, and most people would have no idea he was even running.

2. It might not even be possible for him to get on the ballot in most states as a third-party candidate. Election laws make it nearly impossible for third parties to get on the ballot in many states. In the last 20 years, the Green Party has never managed to get on the ballot in more than 45 states. In 2004, they only managed to get on the ballot in half the states. The Democratic party fights tooth and nail to keep them off the ballot (just like the Republican party fights to keep the Libertarians off the ballot). Because almost all the local and state election officials and judges are Democrats and Republicans, the deck is stacked against third parties.

Finally, Sanders would be viewed as a spoiler, robbing votes from the Democratic party. Most people would feel like they would be throwing their votes away.