I think the only reason this happens is because people associate negativity with a group of people for long enough to tie the negativity to the phrase or word itself. Homeless used to be the sensative replacement for Hobo. After enough time, this new name will just be another tag people can use hatefully.
edit: I must admit, though, I can't see "someone experiencing homelessness" making a very impactful slur.
Montrealer who dumped water on unhoused man's face issues public apology. ( m.youtube.com )