Because the main targets of Koha ILS (not huge public libraries, schools libraries, private libraries) don't need a specific ILL module.
I think the large public libraries and consortia in the U.S. now switching to Koha would disagree with that characterization! Generally speaking, if something is not in Koha, it's because no one has deemed it so mission-critical that they have funded the development of that feature. This could be because they have higher funding priorities, or just because they don't have the money. If an ILL module was something of interest, I'd say the first step would be to put together an outline of what features it would include. See RFCs for Koha 3.2 for examples: http://wiki.koha.org/doku.php?id=en:development:rfcs3.2 -- Owen -- Web Developer Athens County Public Libraries http://www.myacpl.org