Why are there No PURE flying type Only Pokemon

In generation V, there is only 1 pure flying type Pokemon which is Tornadus. It is one of the the Legendary Forces of Nature Trio Pokemon. The other two pokemon are Thundurus (Electric/Flying) and Landorus (Ground/Flying). They all have two forms, Incarnate Forme and Therian Forme. Tornadus' Therian form is based off from a bird, Thundurus' is based off from a mythical serpentine, and Landorus is based off from a beast.