Some of you here may have heard of Justin Trudeau, the Canadian Liberal PM who supposedly epitomizes the values embraced by progressives the world over.
One thing you might not know, though, is that he is also a supporter of various pipeline projects, including the Keystone line that Hollywood environmentalists opposed and Obama vetoed.
Yes, that's right. When it comes to pipelines, Justin is on the same page as Trump.
And do you think this has led to any widespread denunciation of Justin among Canadians who consider themselves progressive? Hardly. About the only people who regard him that way are diehards New Democrats(the Socialist International Party in Canada), who never liked him to begin with. A little less than 20% of the voters last time around. And even a lot of them could probably be convinced to vote Liberal, without any major leftward shift in policy.
Oh, and the Greens probably disagree with him on pipelines too. That's about 3%.
Mutatis mutandis, as long as the US, like Canada(in fact, even moreso) has a de facto two-party system, progressives are not likely to become overly critical of the centre-left party, even just on individual issues. The centre-left party will always be able to point to the right-wing party and say "Yeah, but do you want THOSE guys in power?" If they even have to do THAT much. Often, just the mere presence of the right-wing party makes the liberal party shine, without having to scaremonger the public with the consequences of vote-splitting.
TL/DR: In a two-party system, it's pretty hard to get progressives fired up against the centre-left, since the only perceived alternative is the right.