George Lucas stated in the commentary for the Attack of the Clones DVD that they were clones. I am sure there will be plenty of clone factories. Clones seem to be the perfect warriors. They will not go awal or betray. For example what if the emperor wastes time and money training people only for them to use that training against them in the allience. It doesn't seem efficient. The clones showed in the Clone Wars that they are the perfect warriors. They have all the skill of a trained warrior without any possibility of going awal.