tl;dr : Those who are in positions of power are from the oppressor uppercastes and don't want the oppressed lower castes to be equal to them.

Long answer:

India's Constitution is firm about equality for all castes, religions, and genders. But that chafes those in the oppessor castes and patriarchy who derive their identity from it. The architect of India's Constitution is Dr. Babasaheb Ambedkar who is hated by Brahminism and the patriarchy because 1) he is from the oppressed lower castes 2) he dared to educate himself and architected the Constitution that forces them to be equal to all others 3) he passed the Hindu Code bill that gave equal rights to Hindu women.

Caste allows a person to wield power over others without earning it, just by being born into a oppressor caste family.

Hinduism is a collection of castes. It's communities are based on caste and it's foundation is the caste system. Caste is hierarchial and almost always there's someone lower on the hierarchy who can be oppressed. The sadism of this makes the person forget that almost always there's someone higher on the hierarchy who can oppress, humiliate, and persecute him. This hierarchial system of oppression is so pervasive that thee are sub-castes within a caste which makes every caste itself hierarchial and makes it difficult for people of a caste to unite. In every caste & every subcaste, women are the bottom. This ensures patriarchy is maintained by all castes & subcastes, and gives every frustrated man someone he can punch down to take his anger & frustrations out on. How can any such man who practices hierachial oppression within the confines of his own home take a stand against the hierarchial oppression he suffers from other men out of his home?

Brahmin men are on the very top of the caste pyramid as they authored religious texts on caste and put themselves on top, and their only claim to superiority and an excuse for the many cruelties they've created & imposed on everyone else is based on the caste system. Dalits & Adivasis are at the very bottom, and are at the receiving end of every violence and indignity that the oppressor castes come up with.

We have a law to fight casteist oppression and violence against Dalits & Adivasis called the Prevention of Atrocities against SC/ST Act. But those in charge of implementing this law are from the oppressor upper castes. So the law is rarely applied even though caste atrocities abound.

Caste is a barbaric custom and as Dr. Babasaheb Ambedkar has rightly said, caste must be annihilated.

Because it's inherently tied to Hindu beliefs, and has become culturally pervasive to the point that the attitude follows converts to other systems, including Christianity and Islam which in theory reject it but in practice love them some heirarchy.

Let's just outlaw racism too while we're at it!