Why is it almost impossible to prove legitimate defense in criminal cases? Reyhaneh Jabbari was executed for this reason

Javan newspaper wrote:
Many murder perpetrators, when they appear at the hearing, claim that they were forced to commit murder in self-defense and expect the justice system to consider their defense legitimate.
Contrary to popular belief, the issuance of a "legitimate defense" ruling is a difficult matter that requires the provision of the conditions set by the legislator, and until the conditions are not met, the legitimacy of the defense is void. For example, in the case of Reyhaneh Jabbari, the issue of "legitimate defense" was raised in many cases and it was announced that Reyhaneh had committed murder in self-defense.
However, the law did not give the investigating judges the authority to acquit Reyhaneh of the murder she had deliberately committed, and the margins created by the prevailing text prevented the efforts of the Peace and Reconciliation Commission of the Criminal Prosecutor's Office to satisfy the relatives of the deceased.