Trump is one of the most moderate Republicans on most social issues (abortion, lgb, criminal justice etc.)
He is the most moderate on entitlements (constantly promises to not cut medicare, medicaid and social security) contrary to every Republican campaign in the past (remember Paul Ryan?)
He is a "moderate" on foreign policy (not a Cheney/Bush war hawk, not a 60's style pacifist)
I could go on but I think the important point is; every point he wants to make, he makes in the loudest most wild way possible and people who aren't disposed to vote for him anyway see that as "radical". The correct word IMO is "crazy" or "wild".
Voters who are in the center or can swing either way see him as promising fairly conventional things but in a crazy tone. Maybe tone doesn't matter as much to them