Not to Trump bash, but I think the candidates feel "if Trump can win, anyone can win." Trumps modus operandi in past election campaigns was to show up early on, rattle his saber, but drop out once the campaigns got serious. He used them more as a PR campaign for himself to make a name for himself and get headlines. It wasn't until the last election that he seemed almost flabergasted that folks were starting to rally around him, and the republicans decided to just roll with it. And, the Hillary supporters were so confident Hillary would get elected they forgot to go out and actually vote after they spent so much time on social media saying she was a sure-win.
Since Trump has treated the presidential campaigns / elections as a joke in the past, and suddenly finding himself elected president (irony)... other folks just go "gee, if THAT guy can get into office, then I'm a sure-win!"
But, Trump won because he was voted in as a radical "solution" to help balance out the inbalance people see in this country. We keep seeing the US spend more and more on foreign aid while peoepl in our own country are starving or going without jobs. We see more jobs out-sourcing while people in our own country are jobless and wages stagnate. We see immigrants (documented and undocumented) coming in and working jobs while people are jobless. Trump stood up and said he was going to change all that. There were lots of people upset at how things were going with their lives, he made statements they could believe in, so they voted him in. The irony is that most of these people only ever vote in presidential elections. They don't want to spend time following all politics; they just want to vote on "the big one" and think that will change everything. Meanwhile, they neglect senatorial, house, mayor, governor, etc elections that actually do have major impacts closer to home on their ways of life.
Trump used to make a mockery of our presidential campaigns, now suddenly he's president. It's like seeing a guy making fun of something and not trying hard suddenly being put to the front of the class as the leader of the very thing he used to mock. So, that sends a message to other people that "if this guy doesn't take it seriously, but got to be head of class, imagine how easy it will be for ME to win?"
But, I think the thing these candidates aren't picking up on is that the United States political view has shifted from ultra-conservative, straight-laced, apple-pie politicians to more aggressive, vocal politicians that get up on stage, grand-stand and make big statements about how they're not going to put up with such-n-such anymore or they're going to radically change such-n-such... When everything is going great in peoples lives, they want very toned-down, apple-pie politicians. When people are upset with how things are going, they want more vocal, podium-pounding leaders that seem more emotional and fighting for them. The latest candiates throwing their hats into the ring are all these straight-laced folks thinking they're going to have a chance. They won't. There's still a lot of upset people in the country who still want someone like Trump in charge.. someone that acts like nothing can phase him, unflappable self-confidence, arrogance, pride, etc.. they don't want mellow politicians talking about politicalese that's good for C-SPAN. They want a person like Trump that stands up on stage, makes a scene, talks in clear language people can understand, and puts on a show of leadership for them.
So, unless someone more outlandish then Trump shows up, these mellow real politicians won't stand a chance. There's still tons of Trump supporters, and they'll show up in force to the polls again for re-election. Especially since Trump could now brag that he's had 4 years on-the-job training... where-as all the other candidates are basically applying for a job none of them have ever had in their lives. (It's like interviewing people for a CEO position, and you have one person that has had 4 years experience as a CEO while all other candidates have only been directors or managers.. they have no clue what it's like to be a CEO. Chances are good the person with CEO experience gets hired again over the others.)