To offer an out-of-story explanation:

The Crystal Tower is mandatory now, and the writers have not been shy about using what they can derive from it as a building point for 5.4's content. By contrast, Odin, the so-called Dark Divinity, is entirely optional.

Also, I believe she uses her Dissidia NT model, so there's a good chance not having to create entirely new assets from scratch played a factor in the decision.


As for other possible options:

There is no single being—sans perhaps Zodiark—that embodies Darkness as well as the Cloud of Darkness. The literal personification of the flood of darkness, and the void itself, that consumed the Thirteenth reflection.

Nidhogg is less about Darkness than he is about Blood and Fire. He's also not a primal, so there's little reason to go for him over something more fitting. Tsukuyomi's powers are tied to Moonlight, and Tiamat... was never fought, so we have no idea what she's capable of.