Why?
Good question. Trump people keep saying it, but without any evidence. Seems to me this is the year almost anyone can beat either Bernie or Hillary. Bernie’s an acknowledge socialist and Hillary will have been referred for an indictment by election day.
Because he's not very charismatic.
Because he appeals only to the base.
Because Americans have always been wary of sectarian religious candidates.
Because Hillary is ruthless and he doesn't have the guts to get down in the gutter and fight dirty.
I think Trump is the only guy running who can guarantee a GOP win. Cruz and the rest who appeal only to the dwindling Republican base need Hillary to make plenty of mistakes or be indicted. Only Trump will hit Hillary hard while appealing to swing voters, minorities and the working class to build a new patriotic GOP coalition.