Trump is still around because there is a large base of people who buy into the stuff he says.
Even more, he is still around because he sells papers (well, TV advertising). Even people who don't support him can't help but turn on the TV to see what wacky idea he has come up with today. The media knows this, so they throw him in our faces every chance they get. If he wasn't such an entertaining spectacle, the media would have dropped him months ago and he would have been stumping in half-filled halls to a dwindling number of supporters while the news focused on the other candidates (probably trying to dig up dirt on marital infidelities or contrast a candidates current policies with a statement he made in high-school).
Of course, Trump is well aware of this too, so he keeps saying ever more outrageous things just so he can keep making headlines. Not only does this feed his huge ego, it increases his visibility and makes him seem a viable candidate. Many people refuse to vote for somebody unless they think there's a chance that person can win, and with Trump in the news all the time, it makes him seem more popular than he really is (of course, eventually this becomes a self-fulfilling prophecy).
Trump is one of the greatest political trolls ever, and his success is largely because the news is so addicted to advertising dollars they can't help but feed the troll.
What really terrifies me is that next election, other politicians are going to take note of Trump's success and are going to follow in his footsteps until eventually we'll end up with somebody like President Dwayne Elizondo 'Mountain Dew' Herbert Camacho.