Answer
Due to destructive interference.
Work Step by Step
We know that when light is reflected from the upper surface of the air film, it is reflected at a rarer medium and hence has no path difference. On the other hand, the refracted ray has a path difference $\frac{\lambda}{2}$. Thus, the net path difference between the light ray that is reflected from the top and bottom of the air is $\frac{\lambda}{2}$. Consequently, we have the half-wavelength out of phase and destructive interference occurs, which is why the central spot in Newton's ring is dark.
