I don't know why exactly.
But I want to give it a shot: from my own experience I have noticed that communis doesn't like repotting. Which means in practice, that soil is almost never changed for new soil. With akadama, there's the problem that it breaks down over time and becomes soft clay.
Once the akadama is soft clay, the soil becomes too dense for juniper roots; they suffocate and die.
In the wild, you never see communis grow on clay soils for that reason, I believe. Communis does well on well-draining soil, and akadama drains well.. but only for +/- 3 years.
There will be akadama stuck to the root base after some time, and touching the root base on communis seems to be a killer for them. I did a bare root, a half bare root, and a 1/5th bare root repot this year. And only the 1/5th bare root survived.
From all junipers I've handled this year (scopulorum, chinensis, chinensis blaauw, phoenicea, chinensis stricta, procumbens nana and communis) the communis seems to be the most sensitive of them all. It looks like they just don't like living. Where most junipers take 1-3 months to die or show damage, communis takes about 1 week to turn brown. Other junipers keep branches alive when there's just a small crack, communis kills the branch.
Maybe someone more experienced with communis or junipers in general could have a better explanation. I'm taking a guess here, based on the main reason to avoid akadama.
I have communis seedlings in akadama, they have grown 0 centimeters since they popped up. Communis in regular horticultural seedling mix (potting soil + turf + sand) are 3-4cm in height.