Because instant death effects are a powerful nerf to his regeneration ability (if he fails a save against an instant death effect, his hit points drop to the point where he can be wished dead)
If I made him immune to instant death effects, the only way to kill him would be to melee him down to his hit points +10 subdual damage, while he's regaining Con mod points every turn, has DR, is immune to critical hits and precision damage, can save against anything, and can't be stunned. (And he has maximum hit points, which includes 120 hit points from this class alone)
So yeah, the instant death is there to give a glimmer of hope. Note that the tarrasque is not immune to instant death effects either (they also drop him down to -10 so he can be wished dead). If I gave him immunity to death effects, he would be harder to kill than the tarrasque. That's too powerful.