Because of the finality of it, it's a bit different than impulsively cutting your leg, or taking an overseas trip.
It can be a good decision impulsively in certain circumstances, but they tend to be when something so extreme has just happened that all hope is lost. Like if you're in the trade center when the plane hits and the fire is about to engulf you, you impulsively jump out to your death. It's the right call. It also captures what I think should be the necessary preconditions for impulsive ctb - all hope is extinguished, the only potential future from here is worse, and it's coming fast.
But most impulsive decisions aren't so clear, and they're usually poor ones. If your partner dumps you so you run to the nearest tree and hang yourself because you're sad, that's impulsive, but it's a terrible decision.