In the 2016 US presidential election, Donald Trump is (as of the 20th of Jan 2016) leading the Republican opinion polls, with almost twice the number of points as Ted Cruz.
What makes him so popular?
There are multiple reasons, why Trump is succeeding, and most of those reasons boil down to the fact that he's an interesting candidate.
Beyond simple name recognition, there is the amount of media covering them. Practically any discussion of the presidential race talks about Trump. And everyone knows the basics of his positions:
What exactly are the three most important issues for John Kasich, Chris Christie, etc.? I might manage to answer that for candidates like Rubio and Cruz. For most people, even if one of the other candidates is actually a better match, they don't know.
Trump's rich and owns a business, which gives him the appearance of competence.
Some of this is that he has chosen his issues in such a way that they sound bite well. The media tends to repeat them frequently, even if only to criticize them. But many people feel that the news people don't have the same priorities as they do. So even critical media coverage helps Trump.
He's the only candidate in the race running on populist issues. Muslim ban. Against free trade (although he says he's for it if you use the words free trade). Until now, Cruz has had competition in the evangelical and libertarian spaces from Paul, Santorum, Huckabee, and Carson--perhaps even Rubio. Rubio competes with Kasich, Bush, and Christie and possibly Cruz. Carson and Cruz have a little bit of overlap with Trump, but for the most part, he's had his lane all to himself.
And of course, remember that Trump never has done that well. His high water mark is about 40% in polls. That may be twice that of any other candidate, but it is still well short of a majority. He tends to either be a first choice or not a choice. So as candidates drop out, their supporters will tend to go to other candidates. In three person races, he has been closer to even (e.g. this new poll). In two person races, he loses (e.g. Trump vs. Cruz).
The appearance of Trump dominance has been created by the artificial nature of polling coverage. They tell us national poll results but don't reflect the actual way that primaries work, with multiple rounds that reduce the number of contenders as they go. They ignore statistics that favor other candidates. For example, both Carson and Rubio have much higher favorability ratings.
And of course, it may not be so anymore. The media coverage has shifted since Iowa. Rubio and Cruz have been getting more coverage. One poll has Trump down to a four point lead over Rubio and Cruz.
I was at a lecture last week and the speaker made it so simple:
Donald Trump says replacing Americans with foreign workers is wrong.
Trump is in the lead. DUH.
The speaker pointed out that, just like the attacks in Cologne, the rape in Sweden, most newspapers refuse to print to print stories when Americans are replaced by foreign workers because it is politically incorrect. He noted that ComputerWorld had reported about Disney replacing Americans for six months before the NY Times picked it up. Even now, most news outlets (e.g. Washington post and clearly not ABC news) have not reported the story. Most shocking, he said that until the NY Times reported the story, the Orlando Sentinel had only reported that there were layoffs at Disney but left out the fact that those losing their jobs were being replaced and had to train their foreign replacements.
The point of the talk was that the media turns a blind eye to these issues that affect working Americans in the name of political correctness. So when Donald Trump directly addresses issues with positions that have near universal support, the media has no clue WHY Trump is in the lead.
My objective answer - name recognition. The US is a big place, it's not uncommon for someone living on the west coast to have little or no knowledge of a congressman or senator from the Midwest or east coast. As an example, Bernie Sanders is a Vermont senator, a state abutting my own. I may be ignorant, but until this election season, knew nothing about him.
Trump was and is popular as much for what he isn't as compared to what he is.
The GOP has been affected much more than the Democratic Party by the change in the election finance laws, allowing Super PACs to spend arbitrarily large amounts of advertising money to support their preferred candidates. This has led to the GOP being controlled by just a few people, e.g. the Koch brothers have a lot of influence behind the scenes. This subverts the democratic process, leading to a backlash among voters. But because the billionaires are not all that visible themselves, only the the puppets on the strings they control are visible, the anger of the voters is directed at these puppets.
The new candidates who support a different direction for the Party (e.g. the Tea Party candidates) quickly fall prey to the influence of the billionaires, leading to the GOP voters to get even more frustrated. Note that they get frustrated not because they see that influence, rather because the candidates they voted for end up not performing well. When asked directly, most GOP voters would actually be against laws that would limit the influence of billionaires.
The failure of the democratic processes to steer the GOP in the right direction has thus led to a build up of an enormous amount of negative energy in the system. The bomb finally explodes when someone like Trump who is a billionaire himself comes along, who is able to address the enormous anger that has been build up over the years but who is able to fund himself.