Mostly because the use of lead in fuels is something to avoid at all costs. I know a lot of GA people were VERY happy to hear that they approved G100UL. Leaded fuels aren't just toxic, but they also cause issues with airplanes such as lead fouling.

AVWeb did a couple videos on the subject here:

https://youtu.be/9F-WngVMJBQ?si=Qb_IYu4QwZlOTDCv

https://youtu.be/ovJBJjZTjsk?si=f2OwwZMmuEUTx6wL