The party choise to cost themselves hundreds of thousands of principled, passionate, committed grassroots activists and possibly millions of new voters(though 80% of the Sanders voters who went to the polls in '16 voted for Hillary) rather than making anyone new welcome in the party and rather than acknowledging there was any validity to the Sanders message- it's the same mistake the Dems have made with every runner-up insurgent campaign for the Democratic nomination since 1968- they did it to Eugene McCarthy & RFK's supporters then, they did it to Teddy's supporters in 1980, they did it to the Rainbow Coalition in 1984 & 1988, they did it to Kucinich and Dean's supporters in 2004. The one time the insurgent candidate did win the nomination, George McGovern in 1972, the party chose to refuse to get behind the insurgent nominee and deliberately caused a totally unnecessary landslide defeat just to send the message that insurgents and challengers of the status quo were never going to be made welcome in the party, unless they gave up and became the thing they hated.