Leftist have lost their credibility. In modern world, India has grown to a postition where it does not need any policy of the left. So, when you lost your credibitlity, you need some soft target to keep people distracted.
Because they are hypocrites. They talk as if castism is present only among hindus and not in other religions. According to them, anyone calling himself as a hindu, based on his caste is an oppressor (upper) or oppressed (lower). By creating a false hatred among people, they ensure they have a section always backing them. By means of economical growth, many parts of India has forgotten the aspects of castism but these people ensure it stays alive by keeping people in the dark.
Because they are pseudo secularists. Consider this – any violence against a hindu is not considered as against his religion but if the same things happen to a muslim/christians, it is considered to have happened against the religion. In reality, violence is violence and should be firmly dealt with.