Well, the Reaper Death Seal is a sealing jutsu that originated in Uzushiogakure which was the home of the Uzumaki clan. Which has been wiped out. Kushina had taught this jutsu to Minato, who I'm guessing taught it to the 3rd Hokage. Or it was the previous Jinchuriki before Kushina that did.

Because of this, why would the other lands have this technique since most of the Uzumaki clan is dead?

Also the Naruto Fillers, at least they made sense for the most part...unlike bleach where they're fighting the espada's then you see Ichigo playing soccer for no reason....