I think its because of the definition being set in 2009, then they probably refuse to update it. From their FAQ's:
What is a Heritage Chicken?
The Livestock Conservancy defined Heritage Chicken in April 2009. The abbreviated definition reads: “A Heritage Egg can only be produced by an American Poultry Association Standard breed. A Heritage Chicken is hatched from a Heritage egg sired by an American Poultry Association
Standard breed established prior to the mid-20th century, is slow growing, naturally mated with a long productive outdoor life.”
Click here to see the full explanation.