Was there anything else going on in the late 1980s that might be causing young Black males to be killing each other at alarming rates that might be swamping the impact of legalized abortion over a short time period? Crack cocaine was hitting the inner cities at exactly this time, disproportionately affecting minorities, and the violence was heavily concentrated among young Black males, especially those in gangs.
You wouldn’t argue that global warming is false because for a year or two we had cold winters. You’d want to figure out what effect El Nino has on winter weather and then see whether controlling for El Nino it looks like global warming is taking place. The impact of legalized abortion on crime is a lot like global warming — it is slow and steady and grows a little year by year. Crack is like El Nino, it comes in with a fury and then largely disappears.