Now it is very difficult to say why Sankara wrote like this. We cannot say Sankara is wrong because he was a great enlightened being and respected by many. What we can do, either (1) we can ignore the portion from Brahmasutra which is not relevant to us and to current age, Or (2) we can say "we don't agree with Sankara on that caste aspect, accept the rest related to non-dualism". That solves the problem. There are many saints who have taken disciples even in Vedic period without any caste discriminations. Now it is your choice to whom you want to follow and even to a person how much you want to follow. Out religion was never person dependent.
It is really unfortunate in Hinduism, the book which preaches highest philosophy of God and identity of God with Human being, same book points out that a section of Hindu does not fit for Vedic study. Now we need to change it and already great person like Swami Vivekananda allowed Vedic teaching and study among general mass. The things are changing. We should contribute our share also by studying and preaching Veda.
Everybody can read Veda. Veda declares that as (1) we are Brahman the pure entity, the embodied divine being, and (2) Vedic risis are our ancestors, our parent generation.. So Veda is our birth right for all human being. A single Sankara can't change the divinity. How can anybody become low or outcaste. There are all social disease.
I am quoting two Mantras from Veda which declares equality of all Human being in accessing Veda:
Yajurved 18.48:
rucham no dhehi brahmaneshu rucham rajsu
naskridhi | rucham vishyeshu shudreshu mai dhehi rucha rucham ||
This is a prayer for the intellect/brightness or teja among our Brahmins(Learned
people), among our rulers, Warriors (Kshatriyas), Traders/Vocational people
(Vaishyas) and other working class ( Shudras) and in me too.
Yajurved 26.2:
Yathemam vacham kalyanimavdani janebhyah|
Brahmrajanyabhyagvam shudraya charya ch svaya charnaya ch||
The way I gave this knowledge of Vedas for benefit of all humans, similarly you all also propagate the same for benefit of Brahmins, Kshatriyas, Shudras, Vaishyas, Women and even most downtrodden. The scholars and the wealthy people should ensure that they not deviate from this message of mine.
Hari Om!!
Regards,
-mukti