Asperger's Syndrome has been - and continues to be - poorly understood by those who would typically refer a child for diagnosis and by those who can make a diagnosis.
Diagnosis is often precipitated by the concerns of a child's caretakers. Many parents, teachers, pediatricians, childcare professionals, friends, and family that children come into contact with have only a rudimentary understanding of Asperger's Syndrome, if they're familiar with it at all. People often rely on stereotypes in the media for their information. So, unless a child is a dead ringer for Rain Man or Sheldon Cooper, diagnosis is less likely.
Psychological professionals qualified to diagnose are, sadly, largely unaware of the range of variability within Asperger's Syndrome. While it has gotten better over the years, the problem persists today.
I recently met a psychologist who found it fascinating that I had an Asperger's Diagnosis because it is so rare in women. Considering that it is not an X-linked genetic trait, I am puzzled as to how he came to this conclusion. The rate of diagnosis might be as much as seven times higher in men than in women, but identification of some does not mean that you've found them all.
The reality is that, just as typical women are different from typical men, women with Asperger's Syndrome are - surprise! - different from men with Asperger's Syndrome. Shocking, I know.
Another barrier to diagnosis is the ability to appear to be functioning normally. Many of us can fake it very well. That doesn't mean we're not crippled by social anxiety from an inability to navigate social situations, among other problems. In fact, in some ways the undiagnosed who can remain "under the radar" are not able to thrive the way that a diagnosed Aspie can.
It is getting better, particularly for young boys and for adults who can advocate for themselves. Our young girls experience the most neglect when it comes to diagnosis. We will still see women being diagnosed as adults twenty and thirty years from now.