Vedanta has a rule of entry that no other Indian school imposes so strictly. To found a school a teacher must write commentaries on three texts, the prasthanatrayi or triple foundation: the principal Upanishads, which are revelation; the Bhagavad Gita, which is remembered tradition; and the Brahma Sutras, which are the reasoned summary. A position that has not been argued through all three is an opinion rather than a school.
The Brahma Sutras carry the weight because they are almost unusable on their own. Badarayana compressed the argument into aphorisms of two or three words, many of which cannot be construed without a commentary telling the reader what the missing words are. That extreme compression is why the same text supports readings that contradict each other completely: Shankara, Ramanuja and Madhva each supply different implied terms and each can claim to be doing nothing but reading.
The consequence is a tradition that argues by exegesis. A Vedanta philosopher does not usually set out a system and then look for support; he takes the disputed sutra, the disputed Upanishadic sentence, and shows what it must mean given everything else. The great sentences of the Upanishads, that you are that, that this self is brahman, are the contested ground, and the schools differ over whether such a sentence states an identity, a relation of dependence, or a resemblance.