Because he's theories seem rather quack-ish.
For example, he claims that it is impossible to explain types by what goes on, "in the mind", because it's too difficult.
If we weren't talking about the mind, then what's with this "Please Understand Me" stuff? If the types were as shallow as Kiersey pretends (if you area composer you're ISFP, or whatever it is), then wouldn't understanding people's differences already be blindingly easy... at face value?
Also, labeling Intuitives as Martians is wrong, because you can see many Intuitive types don't follow/believe in that path themselves.
And lastly, he makes a contradiction. He places two, specific, Introverted qualities under the heading of "Intuition", which would just lead more people to mistypings.
There are certainly more, but I won't continue now.
Basically, there are some people who understand psychology and Jungian functions, but have a rather poor understanding of humans. There are others who have a sharp understanding of people, but not of psychology. Kiersey, unfortunatly, understands neither.