Just thinking out loud, but if the perlite is dry and the coco wet the roots would grow towards the wetter side. It's something I need to keep in mind with small plants in large soil pots too, in that case watering from the bottom (letting the medium suck it up) or at least soaking it thoroughly every watering encourages it to use the entire pot. Water moderately from the top has the opposite effect and tend to turn the top layer in a root mass. From what I read you water plenty with run off, but is that drain at the bottom or on the side like a hempy bottle? Perhaps the coco+roots suck the perlite dry before the coco dries out? Based on the size of the plants and the amount of them I did expect more roots. Just shows again what a great job you did though, keeping them healthy with less roots is even harder.