Because the Medicated feed has the minimum amount of Amprolium to help prevent Coccidiosis.I have never used medicated feed but was told that you shouldn't mix medicated feed with other feeds. Not sure y I didn't ask because I never use medicated feed.
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 Starter-Grower. GC