The cheeky response would be why not. The better answer is that having been an avid reader of epic fantasy for more than forty years, I learned that there needs to be a clear differentiation between good and evil in whatever world you craft as a writer.
Sometimes that distinction between good and evil needs to be stark. Sometimes it needs to be more subdued.
Regardless, it needs to be there, because that differentiation oftentimes serves as the crux of the primary conflict in a series.
I had carried the concept of the Talent – my term for the natural magic used in the Realms – around with me ever since I started writing The Sylvan Chronicles, which was before my children were born. It took me a bit longer to come up with its foil. I reviewed several possibilities, let them percolate for a while, asked my wife because if I didn’t she would offer her opinion afterwards and that never goes well, and decided that the Curse was the best choice.
It's important to note that the Curse really isn’t that much different from the Talent. The power functions in the same way. The difference is in its application.
The Curse is a corrupted power, one that uses the user just as if not more so than the user uses it. If you take the risk of using the Curse, then you become the Curse, because you are using that power for yourself rather than for others. Whenever I think of the Curse, I think of the proverb, “power corrupts; absolute power corrupts absolutely.”
That’s my argument and I’m sticking with it. And that’s why I chose to call the world I’ve created the Realms of the Talent and the Curse. A world of good and evil with a bit of grey mixed in every so often. A world where those who seek to dominate must pay a price for that privilege. A price that most aren’t willing to pay, knowing what it will cost them. But there are always a few willing to take that risk as they believe that the rules don’t apply to them.