Because PM Nehruji didn't follow Comrade Stalin's advice on the matter. In India, states were divided according to lingual ethnicity. The same was true in USSR, except they relocated people all across to ensure no single ethnicity dominates a particular SSR. For example, Chechens were transported to Russia, Ukranians to Belarus, Belarussians to Kyrgiztan etc.
We didn't do that step in India, so since state divisions in India was done on ethnicity, the issue of reservation gradually slid into a caste & creed issue. If you want to listen to advise from USSR, we should've done it FULLY.