It's an odd question, I know, but hear me out.

Morgoth, (Melkor), created the orcs out of his petty desire to create a life form of his own. Denied the ability of Manwe to do so, he instead corrupted captured elves until they became brutal, barbaric, anarchistic maniacs.

This is all well and good for Morgoth, who was more of a power mad megalomaniac/ petulant spoiled 5 year old, than anything else.

Enter Sauron as head honcho. Sauron, unlike his boss, was a maia of aule, dedicated to craft, and creation, order and building. He was corrupted by the big M, who used the lure of possessing mighty and beautiful things, rather than aule's path of making and sharing.

But Sauron would still have been, and in fact was portrayed as a dictator. Devoted to his vision of imposing his order upon the world, arranging all things to his desire. One would think that his armies would be made up of extremely well trained, well equipped, and incredibly well disciplined troops. None of which describes Orcs very well.

So was it a question of convenience? Take the forces that are already willing to follow you? It seems a lazy path for a "grand builder and schemer".