[PeterisP]

Quantity of people and the time involved.

This case is temporary, and hopefully short term - homeless people need a permanent, long term solution. Furthermore, for people like legal US residents who are temporarily locked away from their home, shelter generally solves their problems as they need just shelter; while for homeless people it wouldn't solve their root cause of homelessness which generally comes out to either unemployability, addictions or mental illness.

This case also involves much, much less people that the homeless population. In essence, they're choosing to help a smaller problem where they could make a difference instead of a much larger issue that's intractable by those means.