Palin was certainly even less qualified, but thats not important. She was a calculated (and despertly taken) risk, that didnt pay off. Female, white, ultra-conservate with common-sense, "normal"; someone who appeared to be like joe sixpack. Someone the average working class person might be able to identify with. There was at least the chance that she would be able to mobilize different conservative voter groups.
Cruz is nothing like that. He doesnt have an "intellectual" image like e.g. Ryan/Paul and he also isnt able to connect with normal people. Too much Ivy League, arrogance, smugness and no charisma. He might win in the primaries against someone like Huckabee, but he doesnt have the same pull when it comes down to mobilizing conservative voter groups.
That doesnt mean, that the GOP doesnt pick a nutter as VP. There are plenty who could do this job. Its just that Cruz is particularly inapt for this role.