Reason for not putting your name on the ballot??

[Buffalo]

I understand that Obama's name is not on the Michigan primary ballot.

Voters Face Confusion in Michigan Dem Race | The Trail | washingtonpost.com

....OBAMA'S CAMPAIGN CHOSE NOT TO PLACE HIS NAME ON THE MICHIGAN BALLOT SO AS NOT TO VIOLATE NATIONAL DEMOCRATIC PARTY RULES.

Does anybody know the reason or the rule of the national Democratic Party?? Thanks for any explanation.