When I have asked some colleagues why they don't post their papers on the internet, they have replied that it's too complicated, they don't know how to create a web page, they don't understand html, etc. etc. But it is just not that difficult a task. I am certainly no whiz at internet technology, but I've been creating web sites and posting papers for years. If you are associated with a university, your IT people can help you get set up with a home page, and show you how to maintain it and post your papers, images, datafiles, whatever. Once you get the hang of it, it does not take much time. For me, it is part of what I call my scholarly maintenance routine: entering new references into Endnote, keeping up with the journals, and other things I do regularly as part of being a scholar.
I am amazed at the number of professional archaeologists who do NOT post their papers online. Why do we publish journal articles and book chapters? Certainly not for the money. But whether we publish because we feel obligated, or to share our results with others, or for the glory of seeing our names in print—in all these cases we publish so that people will read what we write. If you post your papers on the internet, more people will read them, more people will cite them. IN technical terms, they will have greater impact. This is good for you as a scholar, it is good for the people who read or look at your papers, and it is good for the profession.
To me, this question is a complete no-brainer, which is why I am constantly amazed at colleauges who do not self-archive their publications.
Right now I am feeling like a technical whiz, since I just figured out how to update my university web page from Mexico (it is so easy to do from my office on campus that I forgot some of the steps involved in setting up the right kind of file transfer systen). The main reason I wanted to do that was to post my latest publication, an article on the context and influence of V. Gordon Childe's concept of the "Urban Revolution." Even with interruptions for watching the piñata and eating a piece of birthday cake (there is a 3-year old birthday party going on downstairs), it did not take much time to figure out the new system.