Well, it's complicated, but basically it's because the creator of Dragonball, Akira Toriyama claims that Pan never had a reason or situation to turn super saiyan. Her debut is at the end of dragonball z, as a little kid, and since Dragonball GT is not based off actual manga, Toriyama only envisioned Pan as a little girl in a time of peace, so there would be no reason for her to become super saiyan. Toriyama has also said that he doesnt exactly know how he would draw a female super-saiyan, seeing that some characteristics of super saiyans, like enlarged muscles, befit males only. Try not to worry about Pan not becoming a super saiyan in GT because GT is not based off of the original timeline and Toriyama's ideas... so it's basically nonsensical filler created by an animation studio. I hope