Why is ‘Housing First’ such a tough sell?

The data is in. The U.S. Department of Housing and Urban Development's (HUD) 2010 Annual Homeless Assessment Report to Congress (AHAR) indicates that "Housing First" is effective at preventing and reducing homelessness. This begs the question: Why is "Housing First" such a tough sell?
http://slohomeless.wordpress.com/2011/07/12/why-is-housing-first-such-a-tough-sell/