Vedic knowledge is the first and original spiritual/religious teaching, (before any other) coming to us in the first and perfect spiritul language Sanskrit.
All other religions are just recent fabrications, by societies attempting to create a cheap religious system that offers liberation for doing practically nothing. (Islam was created by a meglamaniac war lord)
Because the Vedanta Sutra may be difficult to follow, because it asks the practitioner to become austere, and to give up materialism, and to study and practice spiritual purificationary techniques, then people reject it (because they are insincere)
A person will not be concerned to give 10 yrs of their life in the pursuit of their career, but for their spiritual life, they are lazy and therefore want a cheap religion, that does not ask too much from them, therefore we see many flocking to Islam and Christianity.
These cheap religions are therefore subsitute religions, and not bonafide.