Which factor contributed most to the rising number of homeless and poverty stricken people in america?

There are a number of factors that have contributed to the increase in poverty and homelessness.

After the Great Depression the United States had a more welfare model to support the economy, this resulted in development of government housing schemes, investment in infrastructure etc which all created jobs.

After World War II as the United States became the dominant economic force in the world, a more capitalistic model was enforced. By the 1960s, poverty began to rise again.

Up until 1970s manufacturing jobs still provided a huge cushion for the poor but as the United States opened by, more and more jobs started to leave America. Most of the jobs that left were the low-paying jobs for low skilled workforce.
 
Loss of jobs, low welfare benefits, the failure of the private sector to build low-cost homes, numerous recessions as well as the recent Great Recession, have all contributed to the rise of poverty and homelessness.