It's not widely available and the doc doing the study was using an experimental design intended to inform clinical decision making. So, he chose options that were commonly available on the market and widely worn among his patients.

I suppose if his patients had all been Portland area vegan-hipster potheads, he might have included hemp, but hemp is still barely legal in the US (the definitions about THC content limits end up making it a logistical pain in the butt to grow commercially).