Because only in this case did Voldemort even consider sparing a parent. Note that James is already dead and his love is not believed to have any extra power. It

Because only in this case did Voldemort even consider sparing a parent. Note that James is already dead and his love is not believed to have any extra power. It was only that Lily had a rare chance to flee and refused to take it that Harry was protected. (Assuming I haven't mixed up canonical Harry Potter with Methods of Rationality.)