Because they’re convinced he’s fighting against the establishment, so since he’s fighting against people they “know” to be liars, he must therefore be more trustworthy. Add to that the “star power” of being a “successful businessman” and people will think he’s fit for the job. That said, that’s a pretty small, but very vocal subset of his base. A much large subset of his base will vote Republican regardless of who the candidate is, so a candidate just needs a large enough, rebid fanbase to get the nomination and they’ll automatically get ~40% of the vote.
The same exists on the other end of the spectrum (vote blue no matter who). You’ll have a very vocal subset for each popular candidate, and a large group who will vote for their party’s nominee regardless.
Those aren’t the people you need to convince, you should be focusing on those in the middle. There are lots of cohorts, such as:
libertarians - anti-war, pro-civil liberties and pro-fiscal responsibility
greens - anti-war and wants fixes for climate change
single issue voters - abortion, health care, national debt, immigration, etc
Those are the groups that care more about whether a candidate lies on issues they care about than party affiliation. So a candidate needs to be careful about what lies they tell.