Because Kishi ran out of symbols to use.
This would be quite unlikely.
Many authors - particularly the graphical variety - love symbolism both literal and figurative. There is no question as to what influences/inspiration Kishimoto is drawing from.
The question is in which direction(s) he will allow it to be expressed within Naruto.
Izanami, for example, was not what I would have expected. Izanagi rejected death at every turn and fled from it. This was reflected very well in the eye technique bearing his name. Izanami, on the other hand, embraced death and mortality - attempting to ensnare Izanagi within a loose concept of Hades (alternatively, one could simply see it as mortality). While the eye technique created by Kishimoto to represent Izanami does link in to the lore... it seems completely underwhelming by comparison. I would have figured Izanami would have been a temporary nullification of all Genjutsu or a genjutsu that binds the life of one to another (similar to the Shinigami/Reaper) symbolizing more the spirit of embracing mortality/death as opposed to rejecting it.
Kishimoto's implementation of Izanami was entirely different and seemed to be over-complicated for an effect that was really only useful on people suffering an identity crisis or Sharingan-Spiders spamming Izanagi (because apparently there were a lot of Uchiha with the Mangekyo Sharingan, Senju DNA, and enough eyes to make Izanagi a serious problem).
So it doesn't always work to infer that relative importance/function of the things Kishimoto draws inspiration from are a strong indicator of how things in Naruto will work.
Though it does often work to predict what other concepts will end up being introduced as the story progresses. It's just when and exactly what those will be that is difficult to answer.
