Was mentioned in the other thread. Seemingly not much of a progression pathway at Rangers for young players, so Cooper and their representation didn't feel it was in his best interest to sign a pro deal there.
Is a common issue with Rangers and Celtic that they hoover up all the quality, but not many actually get first team opportunities, especially with managers chopping and changing at a relatively high frequency. We've shown we'll give youth a chance over the last few years and with the SK link, made even more sense.