Many think that Bill Clinton became President and beat George H.W. Bush in 1992 because of a single catch phrase: "It's the economy stupid!". That may be true since the economy had slipped into recession under Bush.
While we are not in a recession -- at least not yet -- most people feel that we are. In poll after poll, Americans again are saying that the economy and jobs are the number one national priority; well ahead of anything else such as immigration.
What people see in Trump is his success in business. A man who can create jobs and get things done. He tells it like it is. We're right back to "it's the economy, stupid" all over again. A recent Gallup poll supports this with 56% of Republicans; 35% of Independents; and even 16% of Democrats saying he would do a good job with the economy.
Simply, this is why Trump is doing so well against his rivals. He is the only one of those 17 GOP candidates who is a successful businessman. A multi-billionaire. While, Carly Fiorina was also a business person, many know and remember how she had to be let go as CEO from Hewlett-Packard; a scar on her record. She also wasn't successful in her run against Barbara Boxer for California's U.S. Senate seat. Another scar.
Trump may fade but, for now, people seem to think he would be a strong leader on the very issue they consider to be "the" priority.
It's the economy stupid!: https://en.wikipedia.org/wiki/It%27s_the_economy,_stupid
Polling Report: Priorities: http://www.pollingreport.com/prioriti.htm
Trump Stronger on Economy Than Foreign Affairs, Immigration: http://www.gallup.com/poll/184166/trump-stronger-economy-foreign-affairs-immigration.aspx