You'll see that Trump is actually a "nationalist" - which is very popular after so many years of America being run (to the ground) by those in both parties with a globalist agenda.
But since those calling themselves "conservatives" are actually "globalists," who do not want to bring attention to their total lack of nationalism, they call Trump a "populist" rather than a "nationalist."
I agree on the nationalism issue. A big part of Trump’s appeal is his America-first nationalism. And he is not an isolationist.
My reason for calling Trump a populist though is because his underlying philosophy in not conservative. He identifies conservatism as a successful platform. And so he is committing to govern conservatively.
Populism is not a bad thing. My concern is that Trump’s conservatism could easily go by the wayside as he seeks to broker deals and build a consensus. He is not guided by a deeply-held, philosophically-conservative viewpoint. He is more pragmatic.
I strongly prefer someone like Cruz who is conservative as a matter of principle. I may disagree with him on certain issues, but we share the same underlying ideology. For example, he opposed ethanol subsidies in Iowa even though many Iowans want the subsidies. This is comes out of a conservative viewpoint. (I happen to agree on this issue.) An example where I disagree is GMO labelling. Cruz opposes it because he believes science supports the safety and he does not want agribusiness over-regulated. I disagree because I think the science is flawed and people have a right to know what they are buying. But in the end, he will generally arrive at the right position most of the time because of a philosophically conservative view.
Trump might arrive at the same position, but it will be driven from a more pragmatic and populist point of view. An example of how this might manifest itself in a specific issue is Obamacare. A philosophically conservative candidate is one who would replace Obamacare with a free market solution. A pragmatist, like Trump, might simply replace Obamacare with a more efficient, less wasteful, form of socialized medicine. I am not saying this will happen, just that it is more likely.