Why is graphite not the best conductor

Graphite is one of the softest materials.Diamond is one of the hardest materials.The crystal structures and the bonds between the carbon atoms are different.crystalstructure
Synthetic graphite only comes in the hexagonal alpha- Modification before (honeycomb-shaped flat C-layers in ABAB order).
In natural graphite deposits can be up to 30% in the rhombic beta-Modification is present (honeycomb-shaped flat C-layers in ABCABC order).
Comparison of the two modifications: see below
The honeycomb layers are only very loosely bound to one another. The bond between the carbon atoms is strong within the layers.
  Natural and synthetic diamond crystallizes in the cubic system (cube, octahedron, etc.)
There is a very strong bond between all carbon atoms.

ABAB graphite grid:


Diamond lattice:

Only black fabric samples are known of graphite.The color of diamonds varies from colorless, transparent through various shades to black, opaque, depending on the impurity.The color is attributed to the different bonds between the carbon atoms in graphite (delocalized bond) and in diamonds (localized bond).EnamelPoint
Graphite has no melting point, but becomes plastically deformable above 2500 ° C.Diamond melts at 3800 to 4440 ° C (depending on the source) and a pressure between 127 bar and 130,000 bar. Under 127 bar it converts to graphite.
So far, no satisfactory explanations are available.density
The density for amorphous graphite varies from 1.8 to 2.1 g / cm3, for crystalline graphite from 1.9 to 2.3 g / cm3.The density varies from 3.150 to 3.513 g / cm3.  The density fluctuations are unusual and have not yet been explained satisfactorily.
The thermal conductivity of graphite is about as bad as manganese.The thermal conductivity of diamond is very good (about two to five times greater than silver and five to eight times greater than gold).There are completely different heat conduction mechanisms that are complicated to explain.
Polycrystalline graphite samples conduct electricity like metals (about 8 times worse than manganese - the least conductive metal).Synthetic diamonds are semiconductors, natural diamonds are either very good insulators or strong electrical semiconductors.The good conductivity of graphite is attributed to the bonding conditions within the honeycomb carbon planes. The conductivity of diamonds is not yet fully understood.solubility
Graphite dissolves better than diamond in molten metal (especially iron, but also cobalt, nickel, chromium and platinum).Diamond dissolves more poorly than graphite in molten metal (especially iron, but also cobalt, nickel, chromium and platinum).The differences in solubility are not yet fully understood.