It is laughed at because there is no scientific basis for it and the author's 'knowledge' is laughable. And it's all about anecdotes, not studies.
"there has to be some sense in the way that eating how your ancestors have done"
Well, I am eating as my ancestors have done - a diet rich in meat and yoghurt, with a variety of vegetable dishes prepared in the traditional way with olive oil.
And yet, according to the blood type guy I should be a vegetarian 
"the diet certainly seems to work for many people so who is to say it is nonsense?"
Of course it will "work" if you start with a bad diet and start eating more vegetables, cutting out processed food. That doesn't mean he is right in saying different blood types should have different diets and I should be a vegetarian. He is actually impressively wrong about some of his assumptions.
"Nutritional science is in its infancy, there are contradictory theories on everything"
It sounds like you would enjoy Ben Goldacre's book Bad Science. Don't ask why, just read it. You will see what I mean.