Why do you think this is? Perhaps it's striking the working ages who tend to be out in public areas and have more person to person contact. I suspect we'll start seeing it move into the elderly and children before too long.
This was also true of the 1918 pandemic, one reason it was so devastating.
One of the present theories is that the flu turns the body's own immune system against the body. Young, healthy adults with a strong immune system are therefore more susceptible than children and older people.