It took them over a decade to get just a few grams of regolith for the test. You can't exactly support mature plants in a few grams of soil, particularly "soil"

It took them over a decade to get just a few grams of regolith for the test. You can't exactly support mature plants in a few grams of soil, particularly "soil" that is expected to perform poorly. Especially if you want to test multiple plants and therefore need to further subdivide the meager amounts of regolith they have. Proving this basic initial step will open up the avenue for further research with larger amounts of regolith in the hopefully near future.