I'm not saying they should, I'm just wondering if it's because they don't feel a middle name would fit well with the first or surname, or perhaps because it was stressful enough choosing one name, let alone too! 
If you did give your child a middle name, was it because you specifically liked a second name, or wanted to honour a family member, or was it just a case of "that's the done thing". Just curious, my brain has weird musings at this time of night! 