Just one episode after saying "aloha" to Supergirl, Superman found himself saying "aloha" to Supergirl. While Kara enjoyed having her cousin, the Man of Steel, in National City for an entire two episodes all good things must come to end.
In the "Last Children of Krypton" episode, Clark Kent realized that his trip to National City left his beloved Metropolis vulnerable to attack. The new bad guys in town, Project Cadmus, destroyed a park in Metropolis that left Clark saying, "I should have been here. I should've known."
Clark decided to leave Kara and return to his hometown. And he wasn't the only person to leave Kara, as her mentor Cat Grant also bid her adieu.
Cat said, "I'm like a shark in a tank, and I'm swimming around in circles and I'm moving, but I'm not getting anywhere." So she is leaving, but don't be surprised if you see her make a cameo every now and again.