What do you expect them to do? Force uncertain members of the party to vote Yea at gunpoint?
That would be far more competence than I've seen the Democrats display in my lifetime.
Throw the whole thing away in a fit of pique that they can't get everything they wanted?
*anything
They didn't pass the $15 because they did not have the votes to do it. They did not have the votes to do it because too many purple-state D senators did not think their constituents would approve of supporting it. What magic wand do you expect them to wave to change that?
As strong majorities of nearly every demographic support raising the minimum wage, the problem is not the constituents, the problem is the Senators themselves. Or in other words: they're a shit party.
Electing people who are galaxy brained enough to actually think voting down the minimum wage is a good idea is bad enough, but more to the point the liberal ideology of the Democratic Party ensures it does not wholly matter what the individual senators believe. It only matters what the capitalists holding their leashes are interested in. The latter is far more consequential in my judgement, given how the Democrats behave when in minority vs. how gun-shy they are in majority.
If there had been 61 Democratic senators, they would have whittled themselves into this same outcome.