I am not talking about what the party claims. I'm also not surprised that progressive candidates do well in areas progressive enough to choose them in a primary. Maybe I am wrong. Maybe the tea leaves I read are way off and America is ready to bitch-slap conservatives back to the stone age where they belong. It wouldn't be the first time I was wrong, haha. I just think that all the demographics that Democrats have to appeal to in order to win have wildly varying stances on a lot of issues, and they are often not compatible. Why do you think the GOP was able to win with candidates like Dubya and Trump? (He asked, hoping not to receive the answer he suspects he will receive.)
You're asking some questions that are difficult to answer, and the way I look at them may or may not be right.
The first thing I've come to believe over the last two and a half years is that there are very real limits to what can be accomplished with electoral politics. I think back, not to Bush, but to Obama in 2008, when the nation seemed ready for political change. Obama built a movement of people that were frustrated with what had been happening for the last 8 years. The resource wars, the tax cuts for the wealthy, the environmental plunder, and leveraged that into a huge electoral victory.
The conventional wisdom is that Obama was then stonewalled by the blue dog democrats in congress, hamstrung when he lost the majority in the house, and then finished his terms with his hands largely tied. But there are some problems with understanding.
1. Obama filled his cabinet with establishment type Democrats, not a cabinet that was consistent with his campaign of change.
2. Obama remained close to lobbyists and got lots of big dollar donations from special interests. He was playing the same game behind the scenes.
3. He has followed through after his terms by giving high priced speeches to special interest groups, he hasn't fought for anything progressive or put the weight of his considerable popularity within the party behind any progressive causes.
So you had people that wanted change, voted for change, and then the administration came in and just did things the same way Bush did. That will kill the enthusiasm for the next candidate. Compare that with a legitimate progressive like Sanders. He has moved M4A to the head of the table and explicitly called out the special interests that are blocking it. He has very visibly fought for policies that important to a lot of people like student debt relief and minimum wage hikes. Even if he's not elected, the movement that he built could be powerful enough to push for concessions.