Question

why is cucl a solid not soluble if cucl2 is soluble

Answer 1

Compounds of halides and first or second column elements are highly soluble in water, because each atom readily donates or accepts an electron (or two). Calcium, a 2nd column element, readily gives up 2 electrons to form a +2 ion. Copper is not as willing. The behavior of transition metals such as copper is not as uniform as the first and second column elements. That's the basic difference.

Btw, CuCl2 is actually pretty soluble. The highly insoluble one is CuCl.