I think first thing to address here is the assumption that the trial didn't bring Gentileschi's rapist (named Agostino Tassi) to justice. While his sentence doesn't necessarily conform to our modern ideas of "justice" for rape or sexual assault, Tassi was convicted of the crime. He was sentenced to five months of exile from Rome, and was imprisoned for the course of the trial, although he never left the city, and did get his conviction annulled. The entire prosecution was likely, at least in part, an attempt by Orazio Gentileschi to drag Tasso's name through the mud publicly.

To give a little background, Tassi was a colleague of Gentileschi's father, Orazio Gentileschi, and had been hired to tutor Artemisia. After the rape, Orazio pressed charges against Tassi, and many of the court records survive (you can read the court transcripts in an online exhibit from the National Gallery: https://www.nationalgallery.org.uk/exhibitions/past/artemisia/artemisias-rape-trial).

However, immediately after the trial, Gentileschi herself was married to a Florentine man, Pietro Stiattesti, who took her to Florence with him immediately after their marriage. Sheila Barker argues that Gentlieschi's dowry was unusually generous for her class and time, so this may have been a way of helping to rehabilitate her reputation, which had been damaged by her trial. Gentileschi's departure from Rome also clears the stage for Tassi's powerful friends to rehabilitate him, and get the verdict annulled. Ultimately, the verdict of history has come back to haunt him though, as Artemisia Gentileschi is considered one of the finest painters of the 17 th century, and Tassi is rarely discussed outside highly specialized circles, and when he is mentioned it's almost always in the context of the rape trial (although Tassi seems to have been a grade A creep even before this). Most art historians today rate his work as middling at best.

Sources:

Cohen, Elizabeth S. “The Trials of Artemisia Gentileschi: A Rape as History.” The Sixteenth Century Journal 31, no. 1 (2000): 47–75. https://doi.org/10.2307/2671289.

Bissell, R. Ward. “Artemisia Gentileschi-A New Documented Chronology.” The Art Bulletin 50, no. 2 (1968): 153–68. https://doi.org/10.2307/3048529.

BARKER, SHEILA. “A New Document Concerning Artemisia Gentileschi’s Marriage.” The Burlington Magazine 156, no. 1341 (2014): 803–4. http://www.jstor.org/stable/24242500.