The Upaniṣads speak in many voices, from many communities and centuries, and they do not obviously agree. Is the world’s cause a being or beyond all description? Is the self that dreams the same self that is liberated? Passages can be quoted on more than one side. The Brahma Sūtras, attributed by tradition to the sage Bādarāyaṇa, take exactly this as their problem: the text works through disputed Upaniṣadic passages in order and argues that, rightly read, śruti teaches one coherent doctrine of Brahman.
The sūtra style
It does this in the sūtra style: hundreds of aphorisms so compressed that most cannot be construed, let alone judged, without help. That is not a flaw; it is the genre. A sūtra text assumes a teacher or a commentary. So the sūtras became the trellis on which the whole later tradition grew: to found a school of Vedānta was, in practice, to write a commentary on the Brahma Sūtras and defend a reading of them.
Why the commentaries multiply
The commentaries disagree profoundly while citing overlapping scripture, and the sūtras’ own compression is one reason more than one reading is possible. Scholars commonly place the core of the text in the last centuries BCE to the early centuries CE, with the version we have possibly reaching final form around the 5th century CE; like the sūtra genre generally, it may be the work of more than one generation. The text is also known as the Vedānta Sūtras and the Uttara-Mīmāṃsā Sūtras.