One, there is a large Democratic field with at least four candidates with enough money and/or support to justify remaining in the race for a long time: Pete Buttigieg, Joe Biden, and Sens. Bernie Sanders and Elizabeth Warren. There is also enough time for somebody outside this core four to make waves, with Sen. Amy Klobuchar being a prime possibility if she can make a late move in Iowa.
Two, the Democrats have a system in which delegates are awarded on a proportional basis to candidates earning more than 15% statewide or in congressional districts. That means that a candidate who doesn’t win could still stay in the race long enough to mop up enough delegates to deny any one candidate a majority.
Three, the candidates are appealing to different constituencies. While Buttigieg, for instance, polls a lot better among heavily white electorates, Biden is stronger in states with a higher concentration of black voters. Even Warren and Sanders, who are often lumped together, aren’t necessarily drawing from the exact same pool of voters. Sanders has a dedicated enough fan base and enough cash to stay until the end, as he did in 2016.