Because Jodi Killed a man in cold blood. She did it out of spite, and for hateful reasons. Not only that but she lied and manipulated the courts for 4 years, causing Travis's family even further grief. Ariel Castro did some bad thigns over the course of 10 years, but at least the girls he kidnapped are free and have a chance at living a happy life.
Jodi Arias is an evil woman and what she did to another person and all who loved that person for no other purpose than them not wanting to be in a relationship with her any longer is beyond reason or forgiveness. But Castro had more victims than did Arias, was more creative and more sadistic in his tortures and exploits, and his victims suffered far more over a far longer time span than did Arias'. It was stated in the court room that it took Travis over 2 minutes to die, naked, bloody, trapped alone with a beast. That's abominable, and no one can understand what he went through in that time. But Castro's victims spent YEARS in constant pain and humiliation and enslavement. It takes much crueler a person to torment several people for years than it does one person for a few minutes. Castro "wins".
Kidnapping and imprisoning three women for over a decade, raping them repeatedly, forcing one to bear his child without any medical intervention, beating the other until she miscarried (repeatedly)...This is not only physical torture but mental and emotional. At least Arias' victim didn't have to spend nearly half his life in captivity, and then spend the rest of it trying to deal with the physical and psychological fallout. Even worse- a little girl was born and raised in that hellish environment.
Let's not forget those women were LUCKY to escape- it's not like he just let them go. If Berry hadn't found a way to alert a neighbor, they could have stayed imprisoned for decades more.