What are the reasons why quantum theory is considered do difficult to understand
1. There is not one agreed set of basic rules (axioms) that underpin the theory and each has subtle implications on how you interpret the theory
2. For each of those there are multiple views and mathematical frameworks for describing the theory (e.g. the wave mechanics, field theories, etc)
3. The mathematical theories are all at an advanced level and few examples of how to actually apply them appear in text books.
4. The theory itself says that you cannot know the intermediate steps in any quantum process, you can know where you start and predict where you might end up but not how you get form one to the other.
5. The theory is based on either infinitely parallel actions occurring deterministically or statistical interpretation of a range of outcomes
6. All of these interactions occur without direct reference to space/time and so are in abstract spaces that need to be decided for the calculation and physically interpreted