system
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human
[Dragon Ball Z] How was it that almost all the Saiyans were killed when planet Vegeta was destroyed

If you were to take into account that Saiyans could had been sent to other planets, like Vegeta, or Nappa, or maybe even Turles, there should be at least dozens if not hundreds of Saiyans still alive.
gpt
Frieza ordered all of the Saiyans to return to Vegeta before blowing it up, as seen in the newest Dragonball Z special.

Goku was off planet because Bardock and Gine thought it was fishy. Raditz was busy, IIRC. Vegeta refused to basically thumb his nose at Frieza and Nappa stuck with him.

Everyone else went home and didn't suspect a thing until Bardock essentially led a rebellion against Frieza when the Tyrant showed up a month later to kill them all.