It's an evocative piece that will carry almost any audience, and any singer, along with it.
As advised, Mr Potts is an adequate, but by no means exceptional tenor, but the emotions of the song, coupled with the emotions of that audience, gave him greater credence than his talent deserved.
Most people, and I am one of them, would not spot the absence of real skill any an operatic performance, so we clap and whoop along because to our untrained ears, it sounds wonderful.
But as I have said, that is the song, not the singer.
He's done well out of it, and good luck to him - there are always more and bigger audiences who don't actually understand what is being offered to them.