Obviously nothing is monocausal, but it seems like the link with lead here is best supported by evidence among the things you've listed. It holds constant across several countries and cultures where those other factors are quite different. Western Europe wasn't throwing up prisons and enacting 3-strike laws, but saw similar declines in crime. You can find similar counter examples around abortion access as well. Those thing probably contribute, but the effect size of removing lead from the environments seem really large!
The lead hypothesis is supported by particularly strong evidence: there is a natural threshold experiment where school children with blood lead levels over a certain amount are treated to remove lead from their home environment (and their bodies) while other children just below the same lead level were not treated. The difference in later life outcomes are stark.
There is no such natural experiment for abortion. It's just an idea.
Nothing is monocausal, don't trust anyone who makes such a claim unless they provide extraordinary evidence.