He has a history of declaring bankruptcies in a way that fleeces investors and leaves him a profit. In one instance he told them to be happy because they would have lost a lot more money if he actually built anything. And these were everyday joes he was talking to, who had been convinced to invest with him, not diabolical financial institutions.
He is a profoundly petty and destructive man. The best example i can think of is this is suing a journalist for billions of dollars because Donald said he was worth X amount of money and the journalist printed that he was in fact only worth Y amount of money.
He outsources jobs to China.
He brags about how he uses bankruptcy laws to saddle others with his debt.
There's an amazing documentary that was released showcasing Trump attacking a Scottish (Irish?) village to make a golf course.
He lied about not knowing who David Duke was then refused to condemn the KKK because he said he didn't know who they were and some of them might be good people. He refused to condemn his supporters attacking minorities, saying his supporters were passionate, and was talking about paying the legal fees of someone who attacked a black protestor.
Trump U violated the laws in several ways until finally being investigated as an outright scam.
Clearly xenophobic in his rhetoric: talking about banning Muslims and saying the Mexicans coming to America are rapists and murderers
Trump properties investigated for discriminating against blacks.
You might be happy to ignore all of these criticisms because, hey, he was just making money, but it is perfectly reasonable for people to think he is a piece of shit profiting off the very activities that have financially crippled many Americans and has given no indication whatsoever, outside of babbling, that he is going to change a lifetime of his destructive business practices.
Given everything I typed up, you are being deliberately ignorant if you pretend you can't understand why people think he doesn't deserve to be president and that the president of the United States should have exhibited some (any?) business or political practices helping Americans before he decided to run.
Also I recently read a great article that calculated that if Trump had invested his quarter-billion dollar inheritance in index funds he would be worth ten billion dollars more than he currently is as a result of his business practices.
Edit: here it is
http://finance.yahoo.com/news/why-probably-better-investing-donald-233020366.html