Many of Bernie's supporters, myself included, say they will never support her. Hillary is not popular with independents, and we all know it will be very high turnout among Republicans this time because they hate her. In addition, most independents support Bernie, NOT Hillary.
All the polls show that Bernie does better in the general election:
CNN poll:
http://www.cnn.com/2016/03/01/politics/donald-trump-hillary-clinton-bernie-sanders-poll/