I asked this question of a state appellate justice once. According to them, the reason dates back to before electronic databases. Because a lot of the cases are are simple applications of existing law, there is no procedural benefit to publishing those decisions because they simply reiterate what previous holdings stated. By not publishing those, the breadth of possible cases to cite is vastly diminished, and everyone has an easier time "staying on the same page," so to speak. However, when an opinion had a new holding, or dealt with a new area of law, or thoroughly discussed the law such that it was better than existing authority, those were published so that it would become the new thing to cite. Additionally, it saved on printing costs and it saved firms money because they would not need to buy even thicker legal reporters that were simply filled with "repeats."

This worked great for quite a while; however, the rise of electronic databases kinda messed with it. There's no longer that large issue of printing costs when more and more firms are looking to electronic databases. Additionally, whereas before, the opinions wouldn't be found, they are now freely accessible. Further, there's starting to see issues where, sometimes, appellate courts don't follow the law as they should. In some cases, a party will win that probably shouldn't, and the courts will simply find in their favor and not explain the reasoning. Before, these decisions just wouldn't be found. Now, it's becoming an issue because it is become more common.

As I was told, the some lawyers want to cite to these decisions, but can't because of the State Supreme Court rule disallowing it. They are waiting for an appellate court to cite to one in their opinion before they feel comfortable sticking their necks out. This has led to various appellate courts wanting to cite to unpublished opinions, but not wanting to be the first to do so. That's starting to change a bit, though, as some appellate decisions have started citing to unpublished opinions, but have always dropped footnotes explaining that they are citing to the case as an example of the law in action and not for the law itself.