I'll propose that it can be understood trivially.
What would the inclination distribution of circles randomly distributed in three dimensions about some point? We could generate them by distributing the normals to their orbital planes uniformly on a unit sphere, and call the midplane or the plane defined by $\theta=\pi/2$ the ecliptic. Orbits with an inclination $i=0$ will have their normals pointing "up" ($\theta=0$) and those orbiting the "wrong way" with $i=\pi$ will likewise have ($\theta=\pi$).
Then we have $dn/d\theta = \sin(\theta)$ which is zero for orbits near the ecliptic and increases linearly at first.
Fall off at 10-15 degrees is because the solar system is not random but is both a creature and creation of angular momentum.
If you take that data and project it all back to the inclination axis, I predict that you'll see a roughly linear increase from zero. If there's a dead zone very close to zero, that's because the planets are scattered a few degrees in inclination and have a propensity to mix things up.