Well the first one is obvious. They can't. They might strut about, bob their heads and peck at the dirt in a rhythmic fashion but, that's not dancing. It may look like a dance but, it is just their nature. Maybe it is due to their very tiny brains.
The second reason is a bit deeper. My name is Schickedanz. As you might imagine, I was teased about my name when I was a kid. The list of variations is so long that I would need to fill the entirety of this blog to list them all. The most common was Chickendance. I have to say, as a child I really hated this. The constant teasing led me to hate my last name. One day as a young man, I mentioned this to my uncle who said " you should be proud of your name, it's a good name, it's also my name". I have been proud of it ever since. So, when I started this blog, I decided to name it chickens can't dance as a little act of defiance and to finally stand up to all those childhood memories and wash them away for good.
The third is more philosophical. The plain truth in life is if you're a chicken, you can't dance. If you are afraid, you won't go out into the world. You won't dance in this soup called life. So go out there. Don't be a chicken. Dance!