Johnny Cash  

I don't think there's a reason for it, guess he just liked it. I remember someone asking him about it, in Walk the line. DOn't think he gives a real answer!?

I think it had to do with the time he spent in prison, as long as the was injustice in prison he would wear black

He actually explained it himself in the song "The Man In Black". Here's the lyrics http://www.metrolyrics.co... and here's the video for the song http://www.dailymotion.co...
I hope that helps

maybe because that is how he felt on the inside sometimes, or it cause of the song long black train. I am answering from email so I don't know what others have said but I would like to see what they said or thought.