There are a few observational biases at play here: * objects closer to the Sun are intrinsically brighter (as they are simply reflecting sunlight; their intrin

There are a few observational biases at play here: * objects closer to the Sun are intrinsically brighter (as they are simply reflecting sunlight; their intrinsic brightness follows an inverse-square law[0]) * objects closer to the Earth are easier to see (for a similar reason: the amount of light that reaches us depends on those objects' distance from our telescopes; the combined effects of the above two points means that the observed brightness of asteroids approximately follows an inverse-fourth-power law far from the Earth) * objects interior to the Earth's orbit are hard to see because the Sun is in the way (it's usually daytime when these objects are "up"). So, even if there was a uniform distribution of asteroids as a function of orbital separation from the Sun, an observed overdensity of objects should be apparent just outside the Earth's orbit. [0] https://en.wikipedia.org/wiki/Inverse-square_law https://en.wikipedia.org/wiki/Inverse-square_law See also sibling comment from jcims noting the logarithmic scaling, which would compress large separations and expand small separations.