Why is the book titled A Tale of Two Cities?

The two cities in the title are Paris and London, and the story involves both because the events that occur link both cities and Dickens is using the French Revolution as a cautionary tale.

Dickens named the book A Tale of Two Cities because the two cities are its settings.  What happens in Paris affects what happens in London, and vice versa.  Dickens wanted his readers to make the connection between the events in Paris’s past and the conditions in modern-day London.

Dickens begins by explaining why the years of the French Revolution were the best and worst of times.  He then goes on to make comparisons.

[We] had everything before us, we had nothing before us, we were all going direct to Heaven, we were all going direct the other way— in short, the period was so far like the present period, that some of its noisiest authorities insisted on its being received, for good or for evil, in the superlative degree of comparison only. (Book 1, Chapter 1)

Dickens wrote about the French Revolution because he believed that conditions for the poor were very harsh in England in his time.  He wanted his readers to understand that if things continued as they were, a revolution was possible.  He also sought to help his readers understand how the poor felt.  Like in his other novels, Dickens wanted to create empathy in his readers.

Dickens gives many examples of abuses of the poor, including the carriage that ran over the little boy, the Marquis St. Evremonde’s taxation of the village, and Foulon’s reaction when told the people were starving.

"Does everybody here recall old Foulon, who told the famished people that they might eat grass, and who died, and went to Hell?" (Book 2, Chapter 22)

Dickens definitely portrays the horrors of the French Revolution, and personalizes it by how it affects the Manette family.  However, he clearly feels for the poor.   His message is clear.  If you do not help the needy, they may help themselves.