Because some firms don’t see the benefit of distributing their listings to the widest possible audience.

(hat tip: TechKnowBabble)

An off-line comment in response to this post on Agent Genius sums up the argument against excluding listings from IDX very well:

This is a great comment from you “isn’t IDX use saturated enough where a listing agent may have to disclose to the seller that the listing won’t be sent through IDX? (Especially when a company doesn’t have a dominant web presence?)” and it should be expounded on, if a copyright term is used so often that the copyright is longer valid then the point is made here; if placing a listing onto IDX is the norm then the disclosure should be that it will not appear not the other way around GREAT point.