The difference* of course is that none of those people have real political power, Democrats are not trying to appease them. The far left aren't the force behind a Joe Biden or Nancy Pelosi or Chuck Schumer. Those on r/the_donald are Trump's base, the one's that he tries to appease.
*US centric view only, don't know enough about European politics to have an informed opinion.
*US centric view only, don't know enough about European politics to have an informed opinion.