Because they invited him to run in their Democrat primaries? If he went third party ticket, he would cause her to lose to Trump easily. They wanted him out of the picture and no threat. As a matter of fact, they thought that HELLary was good enough to put him away in the beginning of the campaign, but HELLary is just that bad. On a fair playing ground, Bernie would have easily beat her. That's pathetic. Bernie ought to get revenge by running a third party campaign against her, if he could. I bet there would be enough of his followers that would write him in on the ballot.