In one word.
Since Sanders only became a Democrat in order to run for the White House he cares not at all about the party or its future. One might wish Sanders could care about the future of the nation should his behavior this spring cost the Democratic nominee this fall a needed win.
But alas Sanders seems wedded to a more lengthy Wikipedia entry for a legacy than a united front against the Republicans.
Clinton hopes that big victories in New York on April 19 and five Northeastern states a week later will allow her to wrap up the nomination by the end of the month. But aides acknowledge that Sanders … is unlikely to feel significant political or financial pressure to drop out of the race, even if it becomes clear he cannot win … Sanders must win 67 percent of the remaining delegates and uncommitted superdelegates … through June to be able to clinch the Democratic nomination. So far he’s only winning 37 percent.
Joel Benenson, Clinton’s chief strategist, said: ‘We’re going to get to a point at the end of April where there just isn’t enough real estate for him to overcome the lead that we’ve built.’ Still, any kind of truce is probably weeks, if not months, away. … Sanders is costing Clinton significant time, money and political capital.