Why was Tom so successful in his new position provided by the devil What going on in the country at this time to make people need cash

At the time of Tom' Walker's career as a moneylender (loan shark), the country was going through a time of "public distress," or depression. 

The country had been deluged with government bills; the famous Land Bank had been established; there had been a rage for speculating; the people had run mad with schemes for new settlements, for building cities in the wilderness; land jobbers went about with maps of grants, and townships, and El Doradoes, lying nobody knew where, but which everybody was ready to purchase ...everybody was dreaming of making sudden fortunes from nothing.  As usual the fever had subsided; the draem had gone off, and the imaginary fortunes with it; the patients were left in doleful plight...

Tom Walker made especially good use of the circumstances to further his career.  He determined the extent of a customer's need and charged them based on their desperation; the worse a person needed money, the more draining the terms of the loan Tom gave him.  Because Tom was uninterested in kindness, he was able to make money at an amazing rate.