If I may be so bold, I believe I can sum up, in three main points why progressives are so frustrated right now:

  1. They are on the short-end of a left-progressive vs. Third Way ideological divide with the leadership of the American center-left coalition;
  2. In attempts to not be on the short-end of #1, and persuade the coalition rank and file to join them, they face a massive organizational deficit against the coalition leadership;
  3. Finally, if progressives look to split with the coalition in response to #1 and #2, more often than not they just end up getting squashed for it.

Full explanation on the flip side.