We might begin by pointing to society's desire for convenience.
Children are, in some cases, inconvenient.
Thus, some people choose to eliminate them. We might point to the misconception of many that life begins at birth, rather than at conception. We might argue that the most significant cause is society's lack of respect for life. We might argue that contraceptive failure is the leading cause. Having engaged in contraceptive sex and being surprised by the failure of the contraceptive, the couple decide to eliminate the unexpected consequence of their sexual actions.