According to "The Chimney Sweeper" by William Blake, what is the cause of Tom's suffering?

According to "The Chimney Sweeper" by William Blake, the cause of Tom's suffering are harsh working conditions.

When he was just a young boy, his parents practically sold him so that he could start working as a chimney sweeper as soon as he got a little bit older. It is a very dangerous job, and he has to work from early morning, when it tends to get really cold until the very night when they let him rest a bit. Many children die because of these harsh conditions, and Tom is having quite a hard time himself.