for a few reasons.
1. i don't like mixing media types, so no peat in rockwool.
2. i don't like floppy stems, you can't bury a stem in rockwool, but you can in coco.
3. ease of maintenance once the plants are sexed and big enough for cloning. can dial the feed and lighting way back and keep parent plants going relatively easily.
4. i've NEVER had success germinating seeds in wool. it requires way too much attention and finesse, kinda like peat pucks. if they're too wet, the seed rots, if they're too dry, the seed never does its thing. fuck all that, rapid rooters are like cheat codes for germinating.
don't worry, once it's clone time, wool will make its entrance. this is just the pre-game