Why isn't Sanders releasing all his tax returns?

At some point, you have to ask if he's hiding something. Surely the campaign recognizes the importance of transparency on this issue, and could if necessary fund a tax firm to review his tax firms and make sure that they were done properly. He released his 2014 tax return, so that was a start. By not releasing his tax returns, he's providing cover for Trump who, like Sanders, apparently has something to hide. Clinton, Cruz, Kasich, and many of the Republicans who have withdrawn have all released more tax returns than Bernie.

http://www.taxhistory.org/www/website.nsf/web/presidentialtaxreturns

The only other tax return he released was in 1996 when he released his 1995 tax return.

http://www.factcheck.org/2016/04/sanders-evades-tax-question/

But our fact-checking colleague at the Washington Post came across a 1996 news article about Sanders’ 1995 1040 form. The article, which appeared in a Vermont publication called Seven Days, was headlined “El Cheapo!” and focused on the $1,369 that Sanders donated to charity. The article also shows that Sanders reported a total income of $125,842, claimed $25,372 in itemized deductions and paid $19,563 in federal taxes.