Karma and reincarnation provide an answer to problems of evil, suffering and injustice in the world. Karma and reincarnation say that what you do in one life will catch up with you in the next. If you are evil in one life, you will suffer for it when you are reincarnated. Therefore, people who are born to a low caste or in poverty and who suffer greatly are being treated fairly: they are being repaid for the evil that did in a former life.
Judaism and Christianity, on the other hand, do not believe in reincarnation, so they wrestle more with the problems of evil, suffering, and injustice. They don't say that suffering is necessarily deserved. In the book of Job, for example, a good man is allowed to suffer to test his faith—he has done nothing to deserve the horrors that befall him. Instead of providing answers, God states in this biblical book that His ways are not ours way and that much about the universe is mysterious to us as humans. In the book of Isaiah, the idea of the suffering servant is introduced: this is the image of an innocent man suffering pain and injustice for no reason. This concept is picked up in the New Testament, in the life of Jesus, who is also seen as the suffering servant. He is killed in a horrible way even though he is innocent. His suffering, however, is redemptive: he suffers for a reason, which is to help others to be reconciled to God. Judaism and Christianity introduced the idea that a person can suffer and yet be innocent, and also that suffering can be redemptive. Before these faiths, it was believed that a person who suffered must have done something to deserve it. Before these faiths, no good purpose was seen in suffering.
Because Judaism and Christianity realize the innocent can suffer, they tend to orient themselves to social justice and working to eradicate the evils of the world, rather than simply accepting them. For this reason, and no doubt because I was raised in these traditions, it makes more sense for me to embrace a theology that actively tries to right wrongs and alleviate suffering.