As a non-American, looking at the situation from the outside, I am thinking that when the official nominee has not yet been selected by the DNC, why on earth would it not be possible to consider other potential candidates?
I'm American and I think lots of liberals have a backwards conception of voting. Instead of politicians providing a platform to voters to win their vote - voters are expected to vote for corporatist democrats who have enormous war chests of cash.
The amount of $$$ in American politics is insane.