Salaam bhai. Saddening news indeed. One among other tragic things happening.
A few of them do support Modi and the BJP. The current president of India, Ram Nath Kovind, is a Dalit and was a worker in the BJP Dalit Morcha. But these are betrayers of the Dalit liberation cause. I doubt Kovind would have shed tears when the Dalit student Rohith Vemula committed suicide in Hyderabad University hostel in 2016 because of being persecuted by upper caste university officials.
I believe Dalits in general are aware of the oppression and some of them are active politically. One such movement is the recent Bhim Army, named after Bhimrao Ambedkar the Dalit co-writer of the Indian Constitution. The Bhim Army was founded to do direct action against oppression of Dalits. But the Hindutva governments have acted against them. I quote the
Wikipedia page of Bhim Army :
The leader of the group, Chandrashekhar Azad, has also spoken at the JNU university campus in Delhi in support of the campaigns of the Left / progressive students there.
It is sad that caste oppression exists seven decades after the Constitution expressly spoke against it. Just yesterday I read about a village where if a Dalit touches a biscuit pack at the shop of an upper caste the Dalit has to buy the pack. The shopkeeper won't prefer to be "defiled" by retaining that pack.
Firstly, this is my Google understanding of the caste system :
The Dalits are outside of these four specified castes. They are 'achoot' ( untouchables ).
As to your question, I suppose partly the same non-humanitarian superiority complex that the elites in many cultures have for those socio-economically below. And partly a sense of purity where their own group should not be defiled by the presence or inclusion of the Dalit. This is a tribal concept somewhat similar to how the "Syeds" in South Asia prefer to marry among other "Syeds".