Why was Superman not in the CW crossover event?

This week the CW's DC Comics shows had a big crossover event but I couldn't help noticing that someone was missing. I can't believe they left Superman (Tyler Hoechlin) out of one their most highly anticipated crossovers. What was the point of finally introducing Superman in the second season of Supergirl if they don't bring him back for events like this?

Okay maybe in their defense, there wasn't enough time to get Hoechlin back or he was busy working on another project or DC Comics would not allow them to use the character again. So, they probably thought it was best to not even include him. But this version of Superman is a fan favorite and it just seemed weird not to have him in the crossover.