In Part I, we saw that Pirkei Avot, which teaches us so much about Jewish ethics, spends a lot of time on our anticipation of reward and punishment, as incentives to Jewish practice.
Why does Pirkei Avot spend so much time on emphasizing incentives, instead of the innate morality of Jewish ethics?
I see five possible reasons, which may be operating in tandem here (since none of them explain all of the emphasis):
1) Reward and punishment is key to the whole concept of mitzvot and aveirot, commandments and transgressions. Gd says that right off the bat in Bereishit 4:7, in addressing Kayin (Cain) and admonishing him to improve his deeds.
2) Divine justice shows that ethical behavior is not only a social need, it is also a religious, bein adam laMakom need, very much part of being a good Jew as well as a good citizen. Think Bereishit 18:19 where Gd describes "the path of Gd" as being the practice of righteousness and justice. Think of the first mishnah in Avot, which anchors ethics in Jewish tradition and Torah.
3) Insistence on Divine reward and punishment, and particularly on life in the next world, is a major response to theodicy, the problem of seeing apparently good people who suffer and apparently evil people who thrive. This problem is a major disincentive to ethical behavior (see Kohelet 3:19-22), and so it must be addressed (as it is explicitly in Avot 4:15).
4) Reward and punishment work as an incentive for good behavior, as noted by Therapydoc in a comment here.
5) And, finally, if we present Gd as just and fair, then we have a role model for our own behavior. The Torah repeatedly tells us to imitate Gd, and it even gives us specific examples (Bereishit 18:21) to show how Gd judges carefully and fairly; we are meant to do the same.
There is much more to say, but I'm on the run today...