Because when you destroy the head node element, it calls destructor oа unique_ptr, which destroys the second element that calls destructor of the 3rd element which calls ... etс 1mil times.
So, you have 1 mil nested funtion calls (of destructors). Each function call takes memory in stack at least to store return address (and parameters and local variables as well, if needed). Naturally, stack can not provide such amount of memory. You should redesing code to resolve it. For instance, rewrite destructor of Node class so that it finds the last list element and then destroys it and all other nodes from the end in a cicle, not recursievely.
