I am aware that it is a hotly debated topic on what the true origins of Orcs are. However, assuming that Morgoth did create Orcs by corrupting Elves; why didn't he do the same thing for the other conscious beings of Middle Earth? I know that he corrupted the other free peoples in other ways, such as morally or spiritually. But, as far as i'm aware, he never made a new "race" by torturing and subjugating Men, Dwarves, Ents, Eagles, Hobbits, lesser Maiar, or Skin Changers. Is it because of something inherent to Elves, or was it more of act of convenience?
For Morgoth had long prepared his force in secret, while ever the malice of his heart grew greater, and his hatred of the Noldor more bitter; and he desired not only to end his foes but to destroy also and defile the lands that they had taken and made fair. And it is said that his hate overcame his counsel, so that if he had but endured to wait longer, until his designs were full, then the Noldor would have perished utterly. But on his part he esteemed too lightly the valour of the Elves, and of Men he took yet no account.
I assume corrupting races takes time and effort, and possibly some spiritual power. One wouldn't do it to every race, just those useful or important enough to be worth it. So far, it clearly isn't worth it to corrupt Men.
Of Sauron it is said:
Men he found the easiest to sway of all the peoples of the Earth
Why corrupt when you can influence with less effort?
@Michael Foster has a good answer, but I'd add that Morgoth simply did not have the time to corrupt Men like he did some Elves. The Elves arose very early and lived for a very long time in the starlit twilight -- Morgoth discovered them and apparently captured some and took them off to be corrupted into Orcs possibly spending millennia to do it. But Men -- the "late-comers" the "second children of Iluvatar" -- had not yet arisen and when they finally did arise, they arose in the East far from Morgoth's center of power in the North:
At the first rising of the Sun the Younger Children of Iluvatar awoke in the land of Hildrien in the eastward regions of Middle-earth; but the first Sun arose in the West, and the opening eyes of Men were turned towards it, and their feet as they wandered over the Earth for the most part strayed that way. The Atani they were named by the Eldar, the Second People; but they called them also Hildor, the Followers, and many other names Apannar, the After-born, Engwar, the Sickly, and Frimar, the Mortals; and they named them the Usurpers, the Strangers, and the Inscrutable, the Self-cursed, the Heavy-handed, the Night-fearers, the Children of the Sun. Of Men little is told in these tales, which concern the Eldest Days before the waxing of mortals and the waning of the Elves, save of those fathers of men, the Atanatri, who in the first years of the Sun and Moon wandered into the North of the world.
The first years of the Sun and Moon happened after the Nordor had returned to Middle-Earth to recover the Silmarils. (And by then, Morgoth was pretty busy.)