The one who was on track to winning the nomination before the Democratic establishment millions of voters rushed to stack the deck against him go to to polls and vote for someone else that they liked better.
FTFY
This only happened because Obama told Buttigieg & Klobuchar to drop out in the days between SC and ST. Without that, Sanders would have won MA/ME/MN/TX and still be in the driver's seat.
My point still stands - Biden only won because the Democratic establishment stacked the deck in his favor.