CAB grants Indian citizenship to the non-Muslims of Afghanistan, Pakistan and Bangladesh (Applicable on those who came to India illegaly after 31st Dec. 2014) . And NRC is a registry maintained by the Government of India containing names & certain relevant information for identification of Indian citizens of the Assam state (Applicable to those who intered Assam illegaly after 1971).
First thing first, these two are very different things so don't see them as one. Now the question is why muslims have not been included in CAB? Because these following countries named above are Muslim majority so if they come to India illigaly, they do it for a better living (sometimes terorist activities) and not because they had been mistreated in their own country. Unlike other religion people who have no choice but to run away from there because life is hell for them there.
(Don't comment without reading full piece in the caption.)
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