During the reveal trailer we see a training montage for KAY/O which follows an evolutionary algorithm. Long story short, training data is needed for this algorithm implying that KAY/O needs to have fought Radiants before being woken up on Earth 1. This is further supported by the cut to Reyna in the reveal trailer and KAY/O's voiceline "I won't lose you all again". In short, Earth 2 is a wasteland where Reyna and potentially other Radiants committed countless atrocities and KAY/O was built to try and stop them. Without the data on fighting Radiants however they failed but maybe, with more experience, this time will be different.