Why diamond is a hardest structure

[Goodver]

Why diamond lattice structure is a hardest (?) one?

If it is because each atom connected to 4 adjacent, then why simple cubic, where each atom connects to 6 adjacent is not the case?

And why ZnS having the same geometrical structure is not as strong as, say, carbon, or silicon diamond?

Thank you.

 

[DrDu]

Carbon has a valency of 4, so it prefers to form bonds with four neighbours in a tetrahedral environment.
Zn and S have a valency of only two, though the bonding in ZnS is mostly ionic, which is not as strong as the covalent bonds in C.