I mean, it's because lots of Americans are racist. If you are from outside of America this should actually be totally obvious and clear to you since waves of racist nationalism are pretty much sweeping the rest of the world right now.
You're not the first to make this argument but I don't accept it. Is there any other politician out there saying they want to deport all Muslims and build a wall that the Mexicans will pay for and still be in the running for president? This is a whole new dimension of racism and bigotry and it's one of two candidates for POTUS.
This isn't like the German AFD politician who said refugees should be shot at the border. She's nowhere near becoming the Chancellor, she'd stand no chance. Try as a member of one of the big parties to sell people on building a wall to Poland and make the Polish pay for it. Your political career is over.
The only political system (in a democracy) I'd call more fucked up is Austria, and you never want to be compared to Austria.
Americans downplaying Trump as "just like one of those far right guys we're seeing everywhere", this is dangerous. Don't fall for this.