A derivative is essentially an expression for the instantaneous rate of change of a function. Therefore, when your function has no rate of change, your instantaneous rate of change (i.e. your derivative) will be 0.
This is what's happening in your case:
Notice that even as your x-values keep increasing, y-values do not
Therefore, it has a derivative of 0.
For more info on the concept of a derivative, check out the video below:
Hope that helped :)