Perhaps some of these reasons, ranging from the basic controversy of whether it exists at all through a variety of controversies over treatment and diagnosis. Growing up in the UK I guess I'm influenced by the British opinion described there:

The British Psychological Society said in a 1997 report that physicians and psychiatrists should not follow the American example of applying medical labels to such a wide variety of attention-related disorders: "The idea that children who don’t attend or who don’t sit still in school have a mental disorder is not entertained by most British clinicians."

Although the article goes on to say that more recently they appear to have accepted that it may be a real condition.