Additions to a subject are often made by those confused at the time. The material they add is often confusing to those in the field. Much of it is written in a way difficult to decipher. Over time, it is difficult to reconstruct what they were thinking really. This is not always the case, but often is.
Text and course materials can easily reflect such confusion.
The blind leading the blind is often accurate for research and for teaching.
It is also possible for very poor textbooks with large gaps and little hand holding to persist and even be dominant while much better ones are available.
The Greiner textbooks in quantum field theory are an example of ones that fill in many little steps that are perplexing. American universities continue to ignore these as if they did not exist in most cases.
Many original articles skip important details and so do many follow on texts in this very hard to learn subject.
Quantum theory has been characterized by lazy attempts to find an easier way. These end up at the same problems and the details of the calculations are not attended to in many texts.