Simply spoken, because people were pissed and felt betrayed by establishment.
Just a few hours ago I watched something on CNN where they also asked how it came that he won.
Someone mentioned how the dems previously established trade deals "which benefited wall street".
There is only one problem: Americans are not "WallStreet". The avg. American doesn't give a shit about whatever deals that benefited "Wallstreet" or "the establishment", which Clinton was the poster child, in fact they are so pissed off they "hate" them.
They had it, so to speak.
Clinton didn't offer much to people, her speeches to me sounded often scripted and robotic. She only later started to sound "appealing" after Sanders lost where it was clear that she deliberately echoed what Sanders said to please some supporters. The rest of what she said was scripted shit people hear for 8 years already. Except that REAL, nothing has changed for the avg. person.
Trump won because he masterfully understands to talk to those "average" people, with an absolutely ingenious camapign and brilliant PR like his slogan "Make America Great Again", "Drain the Swamp" etc. Yes this is all rhetoric and most often demagoguery, but it resonated with people.
People did NOT necessarily like or trust Trump and believe everything he said. (I mean many Trump voters outright said 'Well I really don't like him...BUT....) <--- but the idea that the hated establishment would be crushed topped all this...(aka: "well I am ready to vote a buffoon into office, if it crushes the establishment that's already reason enough for me")
Basically: Many folks (and yes of course ALSO Sanders supporters ) had been ready for a revolution...and Trump in this case is the one who'd possibly make such a revolution true. Clinton: No way in hell. She was a really, really poor candidate...so poor that an even WORSE candidate, a buffoon like Trump, wiped the floor with her.