Given the huge dislike of government held by most Americans who work and the fact that people are tired of feeling like strangers in their own country anyone who simply speaks the truth is going to get a big following.
Generally speaking Trump would make a horrible president however the real question is why no other Republican has learned from Trump.
To some extent Cruz has with his attack on the Republican establishments oneness with the Democrat establishment but the media has been ignoring Cruz so most of the folks who support Trump haven't heard about Cruz's actions.
Similarly Walker, Carson and some other Republican candidates are starting to be more outspoken, unlike Romney and McCain, but the media is basically refusing to cover them in an attempt to ensure that Hillary lite, Jeb Bush, gets the nomination.
While Jeb Bush is a good person, unlike Hillary, his position on immigration is out of touch with what America needs. According to a recent study something like 6.4% of illegals vote which is enough to swing election results. If we legalize illegals Republicans will never win an election again.
If you doubt that look at California. It elected Reagan but now even moderate Republicans can't win a state wide office. What's changed? A massive increase in the number of immigrants, legal and illegal, and Democrat attorney generals who don't enforce voting laws.
Because illegals, and 52% of legal immigrants, often depend on welfare they can be counted on to vote Democrat.
In the end either the Republican candidate for President in 2016 will either embrace the massive groundswell of disgust with massive immigration when real unemployment in America is sky high or we'll be doomed to 4 more years of Democrat rule.