Because the DNC, every other democratic candidate and the mainstream media backed Biden.

We’re supposed to believe that a candidate who polled 4th and 5th all winter, who had the softest landing into a primary season in terms of big money donors, MSM support and party favouritism, and still could barely fill a retirement home or high school auditorium at his rallies came out of nowhere and beat a candidate who polled in the top two all winter, who came out of the first three primaries with tremendous momentum, whose campaign set record after record in terms of small donor fundraising and number of volunteers, who regularly drew several tens of thousands of people to his rallies, that somehow his campaign all of a sudden ran out of steam and stalled? He got screwed by his own party.