Why is copper pliable but copper sulfate not pliable?

Copper is a simple metal. It is held togeth by metallic bonds. In this setup, positively charged copper ions sit in a mutually shared "pool" of electrons. The ions are essentially held together by their attraction to these electrons. Since these electrons are free to move throughout the metal, the copper is not limited to any rigid form.

Copper sulfate is a salt, consisting of positively charged copper ions and negatively charged sulfate ions. This forms a definite crystal lattice, with the ions arranged into a rigid, repeating structure. When you apply enough force to the crystal, you will force the positive and negative ions out of alignment along a portion of the structure. When this happens the bonds between the ions, which depended on this alignment, break and the crystal fractures or cleaves.