Marley's ghost is still on earth because he is serving his punishment for all his years of avarice and greed. He tells Ebenezer Scrooge, his old business partner, that the spirit within every man is required to

walk abroad among his fellow men [...], and if that spirit goes not forth in life, it is condemned to do so after death. It is doomed to wander through the world [...] and witness what it cannot share, but might have shared on earth, and turned to happiness!

In other words, then, Marley's spirit did not walk abroad among his fellow men during his lifetime, and so he is now condemned to do this in death. He did not witness the suffering of his fellows, or if he did, he simply tried to ignore or avoid it in the future. With his money, he certainly could have helped a great number of people but, instead, he chose to think only about his business rather than recognizing, as he says now, that mankind was his business. He ought to have concerned himself with other people rather than with making money, and he certainly ought to have been more compassionate when conducting his business.

Now, Marley is doomed to wander the earth for a long time, perhaps forever, witnessing the pain of others that he might have assuaged when he was alive, but he does not have the power any more to affect them. It is his own personal version of hell, but it takes place on earth because earth is where other people are—where people suffer.