For many of the same reasons that secular people of other religions become religious. Sometimes, as you grow older, you realize the need for community, and religion offers community. Sometimes, as you grow older, you need to find something spiritual, and religion offers that. Sometimes, as you grow older, you discover that there is great wisdom in ancient tradition. Each non-religious person who becomes more involved with any religious community is entitled to their own reasons for this move. It is worth noting that, while Jews do not generally engage in missionary activity to attract non-Jews, the Chabad movement, a branch of Hassidic Judaism, is very active in outreach to non-religious Jews and is quite happy if they affiliate with any Orthodox community.