Because the Medicated feed has the minimum amount of Amprolium to help prevent Coccidiosis.
So if you mix it with a Non-Medicated feed you'll lose the protection.
I feed Medicated till Chicks are at least 10 weeks old. So I don't give any treats till after I switch to a Non-Medicated...
When I used to change from Starter Grower crumble to a Layers Pellet, I would mix feeds 50/50 for a week or two until Starter Grower feed was used up.
I also mixed Pellets with Scratch Grains 50/50 and scattered on dry ground for a few days to get them used to pellets.
I give Scratch as a...