The 'explanation' in the article is a little thin:

Why is the packing fraction 55% maximum (in globular proteins)? "The answer seems to be that the packing fraction stops increasing when the protein cores jam or rigidify." Ok, so ...

> "That is, the individual amino acids that make up the protein core couldn't compress any further when the protein folded"

So they can't pack any further because they 'jam'? Ohhh, from the abstract of the paper:

> "... However, important developments in the physics of jamming in particulate systems can shed light on the packing of protein cores. ... Then, we develop an all-atom model for proteins and find that, above ∼0.55, protein cores undergo a jamming-like transition"

Possibly this is related to the need for protein cores to remain relatively 'liquid', as enzymes (for example) need to be somewhat flexible when binding/releasing substrates. A fully 'jammed'/packed core would lead to an inflexible structure with lower ability to ... er... move, bind stuff (I'm handwaving here :) )