The short version:
- they are for different audiences. I think it's likely that someone using the docs will spend the vast majority of their time in one or the other, depending on what they're trying to accomplish.
- they are already quite large, and it would be nice to avoid adding another layer of hierarchy at the top.
- less-technical users are easily confused, and this separation helps keep their attention focused on documentation intended for them, as opposed to wandering into the REST API docs or similar and getting the wrong impression about how they should use pulp.
- it's quite common for other types of products to have a separate user guide vs. technical manual, I think for these reasons. Think computers, cars, etc. that have a user manual vs. technical manual.