Something that's popular is practically by definition not on the extreme any more.
In most of the developed world the Democrats would be considered a moderate right-wing party, and the Republicans a far-right theocratic extremism party (like, say the DUP in the UK, or CHP in Canada).
What you're seeing is a ground-shift in values, and what used to be considered OK is now considered extreme, and is therefore being shifted into the margins of the discussion.
To put it in sharper contrast: if you started loudly expressing the most mainstream views of the 1700s in today's society, you'd get a lot of people trying to shut you up and, failing that, marginalize you. And, probably, they'd be right to do so.
In most of the developed world the Democrats would be considered a moderate right-wing party, and the Republicans a far-right theocratic extremism party (like, say the DUP in the UK, or CHP in Canada).
What you're seeing is a ground-shift in values, and what used to be considered OK is now considered extreme, and is therefore being shifted into the margins of the discussion.
To put it in sharper contrast: if you started loudly expressing the most mainstream views of the 1700s in today's society, you'd get a lot of people trying to shut you up and, failing that, marginalize you. And, probably, they'd be right to do so.