During the 2016 campaign Sanders was notable for the way in which he built a large grassroots movement around his candidacy.
Sanders, through both his 2016 and 2020 Presidential campaigns, has built a movement around himself made up of people passionate about his mission of lessening inequality in the United States. He wants to repair a political system that is fundamentally broken, and has engaged voters who are not usually engaged. Like Obama before him, Sanders' popularity proves he can inspire ordinary people, even those who may never have been interested in politics before. In a time with increasingly dire pressures on the population - climate change, economic inequality - what America needs more than anything is an inspiring figurehead that can unite and lead the nation.
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.
[P1] Sanders' campaigns have inspired a large amount of voters. [P2] The ability to inspire voters to this level is an imperative quality in America's next leader.
Rejecting the premises
[Rejecting P2] Sanders' ability to inspire voters may serve to alienate certain voters.