Outliner programs are a seldom analyzed software category. On the same time, lots of these programs are available. So it seems that outliner are interesting for a larger amount of users. What makes the situation difficult to explain is the huge amount of different software programs. A rough estimation is that at least 300 different software programs were programmed since the 1980s and the amount is growing.
Let us explain the situation from a chronological perspective. The good news is, that over the decades only a few larger applications were popular. The 1980s was the decaded of the MS-DOS operating system. Outliner programs like Asksam, Grandview and MORE for the Apple system were created. The interesting situation is, that for the 1980s these programs had a large size. The more software was around 200 kb on a floppy disc. An outliner program was similar to a database and a word processing a complex application. There was even law case started around the PC-Outline program which looks similar to the grandview software from Symantec.
In the 1990s, nearly all the existing MS-DOS based outliner programs lost it's marketshare. Instead new applications like Treepad and Myinfo were programmed. This pattern is visible in the 2000s as well. New software was programmed from scratch like Notecase, Onenote and keynote. It seems, that the avarage lifespan of an outliner program is lower than 10 years.
Even larger software companies like Microsoft are not able to dominate the market. The Onenote software for example is not the market leader but stands in competition to evernote obsidian and of course the older outliners from the 2000s.
If so many different outliner programs are available it should be estimated that it is clear what an outliner is. This is not the case. Most programs are defining the software category slightly different. Some programs are referencing itself as a two pane outliner program, while other are prefering the term PIM or knowledge base. Other programs are calling themself note-taking app.
From a programming perspective an outliner is a combination between a text editor and a file browser. In the manual of the first popular Windows based outliner “treepad” there was a good description given. Treepad is according to the self understanding a combination of the Windows file explorer with the notepad application. That means, an outliner can be replaced by these single tools. In a minimal case it is equal to use the Linux command line for showing a directory and use a text editor to change the content of one of the files. The outliner program simplifies this task and in most cases an additional full text search engine is provided.
Let us take this explanation as a starting point to get a better understanding what an outliner is about. All the operating systems like Windows, unix and Apple have a directory structure on the harddrive. The user can create folders and store text files in these folders. Such a structure is some sort of knowledge base and most user will store information this way. An outliner program makes this use case more explicit and provides a well looking GUI interface which allows to create more complex folder structures.
Even if around 300 different outliner programs are available, most of these programs are working completely different from each other. So it makes sense that so many software was created. What the programmers have done in the last 40 years is to experiment with different GUI layout and program features. For example, in the years of the grandview and asksam, a left pane was missing. Most of the early outliner programs are working with a single pane window. Since the mentioned treepad program from 1995 the situation has changed. Nearly all the programs since the year 2000 are working with a two pane layout. Some programs have more features while other are very minimalist.
Under the LInux operating system there are text only outliner plugins available for the VIM and the emacs editor. These tools are not able to render images because the assumption is / was that images are not needed.
One possible reason why outliner are a seldom explained software category is, that most users think they have no need for such tools. They are using either the folder structure of the operating system or they are storing the outline into a MS-Word document. And indeed, MS-Word can show on the left side an outline window which allows to navigate in larger documents easily.
On the other hand, lots of users are not satisfied with a simple Word document. They have a need for a dedicated note taking app.