Which is something that pisses me off, more or less. If he endorses a third-party candidate, then it'll mean a free ticket to the White House for Trump, something that he stated that he plans on preventing Trump from reaching the White House. It's essentially a "You're damned if you do, you're damned if you don't" situation.
I love Sanders, and I would prefer him over any other candidates. However, the reality is that he lost and the supporters don't want to admit it. Sanders is thinking logically, rather than what everybody must feel, and that is to make sure Trump gets defeated by Hillary (the only candidate who can defeat him).