Specifically the Xindi and Suliban races. From real world stand point and from canon of the show before ENT. That being TOS, TAS, TNG, DS9, and VOY, we the audience and the characters of each series had never encountered or seen either the Suliban or Xindi races. However both races make up the fabric of the ENT era. Why is that?
Because the Xindi and Suliban were not even thought of by the writers of TOS through VOY.
Also, the Federation is big.