In my experience, the actual reason is probably not copyright, as was suggested. The ILL software likely has a few canned responses, and "this is too big, we don't want to scan it" likely rounds to the reason received. I've also had a librarian refuse to scan a relatively short document for "copyright" reasons, despite the document being in the public domain, though not obviously so.
I hope that's the explanation and a little pushback will motivate them into scanning it.
Over the last year, VincentYu, gwern and others have provided many papers for the LessWrong community (87% success rate in 2012) through previous help desk threads. We originally intended to provide editing, research and general troubleshooting help, but article downloads are by far the most requested service.
If you're doing a LessWrong relevant project we want to help you. If you need help accessing a journal article or academic book chapter, we can get it for you. If you need some research or writing help, we can help there too.
Turnaround times for articles published in the last 20 years or so is usually less than a day. Older articles often take a couple days.
Please make new article requests in the comment section of this thread.
If you would like to help out with finding papers, please monitor this thread for requests. If you want to monitor via RSS like I do, many RSS readers will give you the comment feed if you give it the URL for this thread (or use this link directly).
If you have some special skills you want to volunteer, mention them in the comment section.