Because it Can.
I run everything limited edition because variety is the spice of life.
Running new color ways for patches and probably pins isn't just a way to milk money out of a design (despite opinion). Its a little more complicated than that. Some people like a design they just don't like the color and a good way to allow everyone to enjoy it is to run a certain product in a new color. Collectors also enjoy this because it allows them to collect everything in a series. I call them variations.
If I run a handkerchief in gray print and its super popular, the money made on it allows me to run it in a couple new colors. Its a very circular process. The gray variant is still limited edition. You will not find gray ones in the store again if you missed it, but gold is still available and you can own it. Same design, different color. Think spider-man comic book covers.
Also, because I get bored.
I can spend less time restocking items and more time creating new ones. Lets face it, I get bored easy. I think of something I would like to make into a product and I put my money there, rather then in restocking an item that could sit there for a bit. I don't tie up the stores money, and I get to make new exciting things. Its a win win.
The beauty of the unknown.