Because he is probably one of the most truthful person ever. She is just full of corrupt lies.
Obviously Trump is the honest one in the debates. Hillary lies everytime she opens her mouth
She's lying about everything. Where's her emails? What were they for? Why did she deleted them after a subpoena? Why haven't we seen Huma Abadien since they found her emails? Why did she feel the need to bleach bit the emails? “The parties were communicating in a secretive fashion. The operative word is secretive,” Paul Vixie told Slate. “This is more akin to what criminal syndicates do if they are putting together a project.” Is this any indication of why? Why are the aligations against Trump so easily dismantled? Why have people in every poll switched sides? She landed in Benghazi under sniper fire? No false "So much of what Donald Trump just said was wrong" after being called out on her emails? No, Turns out it was all true. There's no voter fraud? There's massive voter fraud with 1.8 million dead voters registered and millions of people registered in two states. Hillary isn't a liar, she's a pathological liar. Almost everything out of her mouth is a lie
Okay, so everybody who says Hillary is a liar and Trump tells the truth all the time is either just plain ignorant or plain dumb. Trump gives untrue statistics all the time in the debates and he never answers a question directly. Hillary on the other hand answers her questions and is truthful.