Because publishing supplementary material mostly hurts researchers. As a researcher, you care about getting papers published and accumulating citations, and not much more. Saying that you
will publish the material makes the paper look a bit better and legitimate to readers and reviewers, even if you never get around to it. Though to be fair, a lot of venues these days require to submit/publish at least the source code.
Some drawbacks of publishing materials such as code:
- You get random people asking questions about it, taking time away from your research. Not a problem if questions are from fellow other researchers or experts and lead to interesting conversations, but most questions are low quality aka "how do I run this on Windows!??"
- You risk people finding a bug in your research code, making your results invalid. Great for science. Bad for your citations, publication chances, and personal resume.
- You need to clean up / maintain code. Often, there is a bunch of private/company infrastructure stuff mixed with research code. Again time that could be better spent on the next paper.
There is basically no benefit to publishing materials other than Twitter clout and PR. If you think your paper/code can go viral due to the extra materials it's worth it because it leads to more citations.
Also, from my experience a lot of the time it's simply due to loss of motivation. The same reason why people abandon side projects. Researchers typically work on multiple projects, and when a paper is published they are likely already working on their next paper. That makes it very easy to procrastinate on extra materials for the "old project"