First, no one is forcing you to have an abortion you don't want. Anti-abortion activity is for the sake of others, and never yourself. This makes it no less noble of an endeavor for your personal beliefs, but since other people having abortions don't actually affect you one wit, it's hard to see abortion as having the same urgency as police brutality, where people at random are dying daily, and anybody could be the next target.

Second, because their voices are being heard. Between the banning of funding for Planned Parenthood domestically as well as internationally, and continued restrictions to constructively ban abortion in several states, it's clear that progress is being made, so voting is working. This is not clear at all for victims of police violence, where claims have been increasing over time, not decreasing.