Because cotton underwear is arguably the right answer:

"Buy this item made out of this material, bury two of them in the ground" requires no further cutting or measuring, and most people would have to go to the store to buy a cut of cotton fabric anyway.

This particular clothing item also contains a more resistant skeleton of fibers against which to compare when the test material is heavily decomposed. This is how the "10 point decomposition scale" functions, and a single cotton square would not facilitate that.

Finally, it's novel, which entices potential participants to complete the survey.

The photos in the team's actual paper (linked in OA) are hilarious and lovely - real pride there. Citizen science projects have educational value as well as the data.