Why don't we like self-publishing?

Steve Lawson has an interesting post on libraries' bias against self-published literature, even that which is written by relatively popular authors who have been previously published in the standard way.  He points to another excellent post on the topic by Tim Spalding, Getting Real: Libraries Are Missing Books.  This would make a really interesting topic for discussion by, say, Uncontrolled Vocabulary, a conference talk table, or a lengthy article. 

Why do we not carry self-published works?  "Traditional publishing" elitism, taking the path of least resistance (read: laziness), the fact that they don't fit into our standard ways of ordering items, or a simple lack of awareness?  A combination of these factors and more?  Something to think about for your own library and for the profession in general.