It's because his supporters are mostly birthers who believe that President Obama is a secret Muslim.
In other words his supporters support Trump because they identify with his outspoken ignorance, his inability to admit mistakes and his trait of speaking in very angry ego-maniacal tones. These supporters gravitate towards people who are certain even if they're certainly wrong. For many, certainty is a virtue while doubt is a vice.
Now here is something that certainly will not bother Trump supporters although it may interest others; the evidence for the above conclusions. A poll from the Public Policy Polling group on 9/3/15 asked some very good and timely questions. Go to
66% of Trump supporters believe that Obama is a Muslim. Only 12% believe he is a Christian. Yup, Obama is one of them gay-loving liberal Muslim jihadists! Only 21% of Trump supporters believe Obama was born in the United States while at the same time 40% of them actively believe that Sen. Ted Cruz WAS born in the U.N. (Cruz was born in Canada.)
Check that out - birthers were totally aghast that Obama became President since he was a Muslim foreigner while ignoring the undisputed fact that Ted Cruz was indeed born abroad and is only a natural born citizen by virtue of the fact that his mother was born in the U.S., which would also be true of Obama (if indeed he was born in Kenya) since no one argues that Obama's mother was born anywhere but in Kansas!
So why will the Media ignore the above? Why do they almost never mention the fact that Trump, for a while, was birther #1? The answer is simple; because to point this out would be to insult the intelligence and sanity of most of his supporters and perhaps a majority of Republicans, which is a large portion of the Media's audience/customers. Please keep in mind that this does not mean ALL Republicans are willfully ignorant and of dubious sanity - John McCain certainly does not fall into this camp, for example. Oh yeah, is it any coincidence that Trump turned on McCain much to the delight of Trump's supporters? I guess that is the price John McCain has to pay for not being totally ignorant or insane.