I heard a really good explanation of this on NPR. Homeless is a label put on a person, similar to saying a person is a redhead. The implication of saying that someone is homeless is that it defines who they are, that it cannot be easily changed.

Unhoused is more descriptive of the situation that a person is in. This is a condition that can be changed, it isn't who the person is.

As I revisit this and think it through though, it seems like another way of pushing the goal. There are absolutely negative connotations with the word homeless, but the same venom will eventually attach to unhoused as well.