Trump is the nominee cause he's a combination of a huge personality and he isn't a politician. Right now, most people that aren't partisan cheerleaders are sick of slimy politicians, and Donald Trump is anything but a politician. For better or for worse, he's about as far from being "establishment" as you can get which is why both sides of the political elite don't like him.
People don't like him because he's a liar and a con-man, who has made money by cheating others and filing for bankruptcy. He lies about everything, has been involved in over 3,000 law suits. He's is far more slimy than anyone he ran against, and certainly a bigger liar than Hillary Clinton, as confirmed by Politifact which labelled 70% of what he says lies versus 20% for Clinton.
As a legal professional specializing in corporate, commercial and real estate law, we consider a law suit a failure: Someone didn't do as promised, pay as promised, or renegged on the deal. No one makes money in a lawsuit except the lawyers. The time and money spent on filings, court appearances, and lawyers is counterproductive, and usually costs more money than you win. Not to mention you can't be making profitable deals when you're dealing with law suits over past failures.
We also don't like him because he promotes racism, misogyny and hatred. He's a demogogue.