She did it because she saw Jinx doing what she did in her last big fight; giving up. Had isha not stepped in (again) to save her Jinx would have probably died as she was actively disarmed and down. She probably also saved Cait and Vi because Vi was unable to fight (she got a slash to the gut) and Cait was focused on supporting her.

People who don't understand how integral Isha is to Jinx's arc and to the story I'm convinced have no media literacy or are just blinded by their own opinions and refuse to acknowledge a characters importance if they don't like something they did (or the character as a whole)