Probably because they don't want ghetto trash moving in and destroying their property. I grew up in Section 8 so I know firsthand, 90% of the folks who get S8 are no good; they don't take pride in their homes (or much else). It sucks for the 10% who are decent, hardworking people, but that's how the cookie crumbles.
They do not want the government up their ***. I have a properrty in a historic district that I rent out. I could not take section 8 because of the modifications that they would require and the inability to do the because of the National Historic criteria.