Bernie Sanders has an army of volunteers and has inspired a grassroots movement. This grassroots movement would help get out the Democratic vote in a general election, which could swing an election in favour of the Democrats given that over 40% of the voting-age population did not vote in the 2016 election.
Bernie Sanders has a strong policy platform on issues which matter to American voters.
Bernie Sanders is one of the most popular politicians in the United States. Therefore, he would be a good candidate in the general election.
The vast majority of Bernie Sanders' past campaign fundraising has come from small dollar donations (<$200). He is therefore more likely to represent the interests of the US people, rather than corporate interests.