Good question. I suppose that after a certain point the Secret Service has to protect the private information of a president, but I distinctly remember in the Clinton terms a full disclosure of his tax returns. President Obama is clearly 'the man who never was'. We know literally nothing about him at all. He is the figment of somebody's imagination. We just don't know whose.
“I distinctly remember in the Clinton terms a full disclosure of his tax returns.”
This site is a good source for Presidential, Vice-Presidential and want-to-be presidential tax returns.
http://www.taxhistory.org/www/website.nsf/Web/PresidentialTaxReturns?OpenDocument