If the producers and judges just picked the best singers in the various boot camps and elimination rounds, people wouldn't vote as much as they do now, and it would also be a bit boring. The likes of Wagner are kept in, firstly by the rebels who want to turn the show into a farce by voting for him, and are so far succeeding, as they did with the Joe Mcelderry not getting to number one at Christmas thing last year, and secondly because they just assume that other watchers will vote for the better singers.
Its not the be all and end all, it's supposed to be entertainment......