Because of it’s rarity. In the world 1 out of 25 000 is affected by PKU. In the States, 1 out of 15 000 is affected, Turkey 1 out of 2 300. In Finland, however, only one out of 100 000 has PKU. When the chances for the child are 0,00001% to have PKU, it’s not routinely tested on all infants.
20 out of 34 hospitals test for PKU, but most only if both parents are foreign and only three hospital test if one of the parent is foreign. None of them test PKU on every child.