fluid dynamics - Drag force at high speeds

The drag force on a spherical body according to Stokes' law is given by

$$F = 6π\mu rv$$ Where $\mu$ is the dynamic viscosity of the fluid, $r$ is the radius of the spherical object, and $v$ is its velocity.

At low speeds, the drag force is directly proportional to the speed of the object. While at high speeds, the drag force is proportional to the square of the speed of the spherical object:

$$F = \frac{1}{2}\rho v^2C_dA$$

Why does this happen?