People without the relevant background can't judge the content, but they hear the the tone and attitude and they can't imagine using that approach unless they were telling the truth. They just cant imagine being a pathological liar with testicles of neutronium themselves... and so, improbable as it seems, they actually find Wright's spiel credible even though to the rest of us it wouldn't even make for a credible conman act in a movie.
In fact, his utter implausibility works in his favour in another way: it discourages competent people from spending their time discrediting him, and when they do it they find themselves saying something like "wtf. how could you _possibly_ believe this crap. I can't even.", -- not exactly the most effective, even though it's actually a fair response considering the utter absurdity of the situation.
Also, since no one has linked it and to avoid myself being guilty of just giving a "I can't even" response, the bitcoin wiki page on Wright is obligatory reading: https://en.bitcoin.it/wiki/Craig_Wright