The “their” you reference is the Democratic Party. It’s the party’s best chance of defeating Trump. People with better chances don’t fit well enough within the party to get their nomination.
And the only reason the Democratic Party has a say is because the U.S. political system was designed to only allow one of two winners. The system is designed so that any third party offering will fail, and their attempt will harm the party with the “nearest” compatible beliefs.
It’s a shitshow. Built poorly (on purpose to preserve power of the political elites), and litigated to be even worse, so corporations can bribe those elites.