Why? Because it shouldn't be visible, not like that. That image should be a fog or completely black, nothing in between. To see that amount of detail, that amount of fine structure, those beautiful delicate rotating pinwheels, to see galaxies billions of light years away across almost an entire universe is absurd. That mind-blowing image is impossible without a super-finely tuned matter density gradient and alpha opacity function that is absolutely perfect. It is more mathematically improbable than hitting the Power Ball Jackpot lottery five times in a row.
The UDF is one of the many wildly improbable aspects of the physical Universe. It is unique in that it does not fall prey to a rebuttal based on the Anthropic Principle. (I believe that AP is the last, desperate, refuge of the cornered atheist.)
See the YouTube video The Hubble Ultra Deep Field in 3D by Deep Astronomy. Watch this video carefully. (Or just look out your window.)
“By his spirit he hath garnished the heavens; his hand hath formed the crooked serpent.”
Job 26:13