Is it just that it’s a really fucking hard job? How do you protect someone who wants to travel the country and stand in front of live audiences of thousands of people, without getting in the way of the personal experience they are trying to sell?
If you are as controversial as Trump, anyone can be an assassin and the secret service have to somehow separate crazy fan from crazy assassin, in a thousand different venues. The only real option is a bulletproof Popemobile.