The reason that Sanders' movement is so strong is because it speaks to a certain niche of the population - the far Left. This by no means necessarily translate to broad, national appeal. In fact, the intensity of Sanders' supporters may serve to alienate. He scares many Democrats, and as primary voters tend to be more liberal than Democrats as a whole, his momentum in the primaries may not translate into success in the general election.
The very reasons Sanders and his campaign have been so successful may serve to turn off the broader voters.