Mostly it started because the last party PT, had created the biggest corruption scandal on the American continent. Way, way bigger than watergate, and because PT has left Brasil in a gigantic debt.

People that were already sick of corruption saw Bolsonaro as a way out, since he had a hard stand against it, and was supported by Sergio Moro as his minister of justice, a man involved in taking down this corruption scheme. On the economic side, he had Paulo Guedes as his minister of economics, which is seen as a liberal alternative to the last governments.

In Brasil the government is extremely corrupt, and most rich people are public employees that keep getting raises despite Brasil's economy's situation. (Not all public employees are rich, most are not. But most rich people here are public employees. Also, governmental services suck.). For the people that voted for Bolsonaro, the government is seen as the source of most of our problems. So the people that voted were expecting: to bring less governmental interference in our business, lower government expenses so we can pay our gigantic debt, and hopefully eventually pay lower taxes, and get rid of the party ( PT ) responsible for the biggest corruption scandal in the continent of America.