The greatest reason that I see for supporting other candidates is his spending plan and the impossibility of paying for it. The Wall Street Journal has a nice analysis of the spending proposal and the dramatic deficit it would create.
http://www.wsj.com/articles/price-tag-of-bernie-sanders-proposals-18-trillion-1442271511
There are ALWAYS unintended consequences of government action. We should carefully examine the potential pitfalls of any legislation, and Sanders' proposals are full of them.
He will continue to find support among progressives who felt let down by a president they consider too moderate, too friendly with Wall Street and corporate America.