As I've remarked elsewhere, in almost any other country in the world, the people you call 'progressives' or 'leftists' would be regarded as centre-right conservatives.
The Democratic Party suffers from the same problem as the British Labour Party does - that of being too broad a church. The constant pushing and pulling between the Comunist left and the Social Democratic right of the Labour Party had often rendered it unelectable, or ineffectual when elected. For instance, in the last General Election, the 'Brexit Election', the right wing of Labour was for Remain, while the left was for Leave, and Jeremy Corbyn was unable to persuade or force the party to present a united view on the subject. Had he made his own, Socialist, pro-Brexit beliefs official policy, Labour might have won, but the cost would have been rebellion in the ranks.
The same applies to the Democrats, with the tension between centre-right supporters of Sanders and the hard-right old guard who support Biden.