If this is the case then why don't "moderate" candidates win more races nationwide?
Because the activists are the only people who reliably show up to the primaries.
Yeah, isn't that right President Bernie Sanders and Congresswoman Nina Turner and Senator Paula Jean Swearengin
That applies a lot more to the Republican Party when it comes to primaries. I think Democrats are also more cautious in nominating more left-wing candidates because they fear it costs them swing votes. Republicans don't bother that much and nominate extreme candidates.
The media playing the both sides game certainly plays a role in that perception, and also Republicans successfully tying Democrats to fringe leftist positions the mainstream of the party doesn't even support ("defund the police", "open borders", "late-term abortion just before birth" and so on).