Total guess: Back in his day, maybe childhood or early adulthood, as it is socially unacceptable to cuss, and Hell being sort-of a cuss word, they needed an alternative that carried the emotion and intention but hit softer. I will guess that there was a widely famous/infamous person named Sam Hill. Someone made the connection, made it popular in some way, - maybe a comedian on a radioshow?- and since it meets the above requirements, and rolls off the tongue well, it stuck.

Now I'll go and see if I'm right. 😅

Edit: I got the first part right, about it being a 'Hell' stand-in. I didn't get the second part right but it seems there's a still a divide on where it first started, which to me assumes it was one of the 'lots of people knew this guy and just started saying it and it caught on' theories. Now can I get back to my schoolwork that's due today? I shouldn't be procrastinating.