How did scores of homeless come to live at the Convention Center?

The street sweeper arrived at 12th and Arch shortly after dawn. With a broom and a wheeled dumpster, he swept around the sleeping forms on the sidewalk under the arches of the Convention Center. As commuters walked past and dozens on the sidewalk stirred awake, the street sweeper collected bottles and plastic wrappers and half-eaten food - all the detritus of a night on the streets.