[zoky]

Because it’s not that interesting. Bad guy decides being bad is boring, and starts being good? Like, okay, but… why?

[HeatrayEnjoyer]

How is that any more boring than the opposite?

[Apocryphon]

Presumably because we see the actions and consequences attached to him deciding to be bad. If the good is only contained in one cursory final chapter, there's no substance to it. Might as well save it for a sequel.

[longcrab]

Partly because complex empathy for others is built with experience, so the teenage main character eventually grows up.