Firstly, it was but (apart from being discussion of the phrase rather than actually planning to kill him) it was swiftly taken down .

Secondly, at risk of stating the obvious: Parler has self-selected its audience to consist of proportionally many more extremists, they still exist on twitter but not even close to as prevalently on Parler (not least because many extremists on Twitter are going to be on the left, where the dialogue is harder to police because it's much less obvious where the violence starts)