Because Tiamat is the Mother of Monsters, the spawner of Primordials, the goddess of the cosmic sea from whence all things come, she is the terrible, the beautiful..WotC have confirmed this is the "full" Tiamat.
Why is that actually? I mean, none of the D&D lore, no matter the edition supports it, and yet you always make Tiamat one of the most powerful - if not the most powerful - of your designs. So just wondering.
(And I've said before - I way prefer her as the Ur-dragon, instead of a goddess.)
Wikipedia said:She is the symbol of the chaos of primordial creation. She is referred to as a woman, and described as the glistening one. It is suggested that there are two parts to the Tiamat mythos, the first in which Tiamat is a creator goddess, through a sacred marriage between salt and fresh water, peacefully creating the cosmos through successive generations. In the second Chaoskampf Tiamat is considered the monstrous embodiment of primordial chaos. Some sources identify her with images of a sea serpent or dragon.