Orcs have no magic. Warcries.

In Heroes 1-3, the Orcs had a different concept; even though they leaned towards physical strength, they still had some variety. But here, they decided to make them purely warriors, through and through, and in my opinion, that's much better: it adds more tactical diversity. However, the fact that the Orcs in Heroes 6 cast spells just as well as other factions can only be attributed to laziness on the developers' part. They were too lazy to make them unique, and that's all there is to it.