I believe that Blake wrote this poem to decry the state of people in the urban centers of his time. He argues, in the poem, that people in the cities are really badly off in a number of ways.
In this poem, Blake is saying, for example, that people are oppressed. He says that they are, essentially, living in chains. The have manacles on that have been formed by their minds.
In addition, people are in bad physical condition because of their poverty. You can see the marks of weakness on them wherever you look.
Finally, they are badly off morally. Women have, for example, been forced to become prostitutes.
All in all, life in the cities has degraded the humanity of the people who live in it.
This is, I believe, what Blake had in mind when he wrote the poem.