Top DIRTIEST cites in the US

The "dirtiest" city doesn't necessarily mean that the city doesn't take a good care of itself, and the "dirtiness" is not proportional to the size of the city or the population.

(This ranking is provided from Travel + Leisure magazine: http://www.travelandleisure.com/articles/americas-dirtiest-cities)

1. New Orleans

2. Philadelphia

3. Los Angeles

4. Memphis

5. New York City

6. Baltimore

7. Las Vegas

8. Miami

9. Atlanta

10. Houston

What caused these cities to be ranked as the dirtiest cities in the United States?

What do you think the reasons are?