Animal rights activists are drawn to animal rights work for a variety of reasons, so there isn't a single reason for people becoming animal rights activists. However, asking why animal rights activists work for animal rights instead of human rights assumes that the two are different, mutually exclusive or even at odds with each other.
Throughout history, social justice activists have been accused of favoring the oppressed over the oppressor, when justice requires that all be treated with equal respect and protection. Civil rights activists do not work for the rights of minorities instead of the rights of others; they work for justice for all and ask only that no one's rights are trampled.
Similarly, animal rights activists have not chosen to work for animal rights instead of human rights, but are working towards a just world where neither group is exploited. Of course there are situations where humans are exploited by other humans. As many animal rights activists point out, humans are animals too, and animal rights activists oppose all exploitation of human and nonhuman animals.
