[tomr_stargazer]

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

See also sibling comment from jcims noting the logarithmic scaling, which would compress large separations and expand small separations.

[Symmetry]

Those biases exist but don't explain why we see more asteroids between Jupiter and Mars than between Mars and Earth. There really is a much larger concentration of asteroids in the asteroid belt.

[dotancohen]

Yes, I'm not referring to the asteroid belt but to the phenomenon that below 1 AU there is a sharp drop in the quantity of asteroids shown in that map.