I suppose I can make two arguments on behalf of the way things are done.

First, the jury's role is to establish whether or not the defendant is guilty of the criminal act as defined by the statute. Did the defendant commit the crime, yes or no? Someone's reasons, in a case like this, don't really change whether they did the crime or not. They're superfluous to the limited question at hand.

Sentencing is the part where we worry about how much someone deserves to be punished for committing the crime, so mitigating factors are relevant there.

Second, it might be dangerous to the justice system's impartiality and effectiveness to allow a jury to consider other factors when they come to a decision about guilt. The laws are supposed to apply equally to everyone. If juries start carving out unpredictable exceptions based on how sympathetic they feel a particular defendant is, then the application of justice might become very unequal. For example, young black men stereotyped as hoodlums might be treated much more harshly than young, attractive white women who have different, more positive stereotypes. (Indeed, that sort of bias is already a problem even with the current system.)