Off the top of my head:

1) When the rapist has sufficient status or allies to prevent negative consequences.

2) If the victim is of a rival group to that of the rapist. Different tribe. Different 'caste'. Different party within the same tribe.

3) The social rules don't enforce a rape taboo strongly. In many cultures rape is defended by family vengeance and not particularly by 'justice'.

4) The consequences to women don't make 'reporting and punishment' the expected outcome.

5) When 'rape' is defined differently to how it is defined by us. (eg. Wives, dates, underage, those under authority.)

6) If reproductive prospects look bleak the expected payoff doesn't need to be particularly high.