The most interesting thing about this question is that I've never answered it before. It's never been asked, is the other way of saying that. As in, no one ever asked me to answer this question on my blog. I checked. Couldn't find it anywhere. It's almost like it's never been asked, and I've never answered it.
That was me stalling.
Why did the chicken cross the road? I don't know, man. Chickens are fucking stupid. They don't even know what a road is, so this is the same as asking 'Why did the chicken wink at me like that?' He didn't. He's an idiot. Or, 'Why did the chicken just fall over in its own shit?' Again, stupid chicken.
If a chicken crosses a road, it's because the road appeared before it while it was aimlessly wandering in a direction.
Now the philosophical component. I think this 'joke' is supposed to make you think deeply about the answer being inherent in the question, which is an interesting thing to explore on this particular blog, based on the format. The reason, in its basest form, that a person moves from point A to point B is because they wish to be at point B. Any other reasons are more subjective and exploratory and must take into account external factors like motivation. And as we've discussed, chickens be idiots.
Another way to look at it is: Why does anyone do anything? How can we assume the basest form of understanding of the universe with so many variables present? The answer may very well be that we have no idea why a chicken would cross a road. We can never ask the chicken, therefore making its stupidity a perfect metaphor for our lack of ability to understand the world around us. The chicken's motivations are a mystery, just like everything else.
I feel like I'm tumbling down the chicken hole, here.
Short Answer: Why did the chicken cross the road? Existence is a mystery and we're all going to die.