The non-kosher ones may have additives or other ingredients that are not kosher The non-kosher ones may be produced on a production line which shares components with a non-kosher product. read more
If they contain dairy products, they would not be kosher with meat. It is written not to boil a kid (baby goat) in it's mother's milk. Stricter traditions make very wide allowances to make sure such a thing would never happen, to the point of waiting several hours before combining these foodgroups in their stomach. read more