Must the Upanishads be 'difficult'? Actually, they are rather plain and direct. They do not construct any complex system of ideas and beliefs. Nor do they build up any elaborate picture of the world. Instead, in some what brief and uncompromising language, they ask what is plainly and simply true: beneath all the complications of our uncertain beliefs.
As a result, the Upanishads were kept traditionally secret and inaccessible. They were hidden behind a forbidding reputation: as teaching an esoteric and mystical doctrine, to be kept away from all but a few special initiates.