why do democrats keep saying "XYZ is too radical to be President"?

Whenever a candidate gets mentioned to run for President, such as AOC or Bernie Sanders, I see a ton of Democrats shutting it down and saying no way, they are too radical, the moderate would never vote for them.

Donald Trump is far from moderate. Dude is definitely an extremely radical candidate. The American people do not care. It seems like they want something radical and different vs the status quo, whether it be from the left or the right.

So what gives?