Because today it requires tricks and mutations, whereas all archaeopteryx had them as a matter of course. Even creationists manage to learn that chickens have beaks and not teeth.
Moreover, the fact that it is at all possible to "trick" a chicken into growing teeth is another classic evidence of evolution. Chickens have teeth in their ancestry: it's written in their genes as well as in their fossil history. Stephen Jay Gould even titled one of his books Hen's Teeth and Horse's Toes.
It doesn't require "mutations" at all. The birds all have the genes to grow teeth. You remove the DNA methylation that prevents them from being expressed and you'll get a bird with a mouthful of teeth.