You have to understand Americans and American history to understand why. Americans, by and large are very polarised politically - you're either one or the other, to the point it even influences where you live. A lot of Americans never leave America so their only window into the outside world is what Fox or CNN tell them. They don't really experience how it could be. Of course this isn't the case for everyone, but the more progressive states seem to be those that are international hubs that bring in other influences.
They're terrified about someone coming to take it all away and Trump prays on that brilliantly. Listen to any of his speeches, you can pick out the buzz words. The immigrants are coming for you, the women are getting too big for their boots, Europe needs to be put in its place, I'm a dictator, I took a bullet for democracy which is under threat from all of the above. They're coming to take everything that's yours and you need to fight so they don't.
We see America as being on a level playing field with Europe but you forget that when Europe had decided to stop warring and settle down, they were just getting started. A lot of groups, factions and religions whose views were no longer being tolerated in Europe moved to the new world to be free to live their lives the way they wanted. It's not that many generations ago that they were having it all taken away from them and not enough time has passed to make them complacent about that yet. Think about how things were here about 150/200 years ago and it will start to make a bit more sense.
I'm not saying we're perfect in any way but half my family live in the US so it's been interesting to see the Trump situation develop.