Short answer:
Think of the situation in a spherical cooridnate system, with the point of reflection in the water surface at origin and the $z$-axis parallel to the still water surface and pointing in the azimuthal direction of the sun.
As the sun gets lower, both the incoming and reflected light rays becomes closer to parallel to the $z$-axis. Imagine rotating the water surface by small amounts in the $\theta$ and $\varphi$ directions respectively and think of the effects that has on the direction of the reflected light.
As the light rays get closer to parallel to the $z$-axis, the effects of rotations in the $\varphi$ direction have smaller and smaller effects on the direction of the reflected light. In the extreme case of the sun at the horizon, rotations in $\varphi$ have no effect at all. 797
This is because all values of $\varphi$ represent the same point for $\theta=0$ and $\theta=\pi$.
Longer version of the same answer (more math):
I find it's helpful to think about this in a spherical coordinate system:
The coordinate system is chosen so that the $x$-axis is orthogonal to still water surface, and the $y$-axis is orthogonal to the incoming sun light. Imagine the reflecting water surface at the origin, and 3 vectors on the unit sphere. The first vector is the normal of the reflecting surface, which we will write on this form:
$$ \boldsymbol{r}_n = (1,\, \pi/2 + \delta\theta_n,\, \delta\varphi_n) $$
It's basically pointing in the $x$ direction, except for small variations due to waves represented by $\delta\theta_n$ and $\delta\varphi_n$. 586