The same reason that when you were younger your parents told you to count to ten when you were upset or angry. If you are angry, you look for a quick outlet for that anger, speaking is the most readily available medium. Commenting on the internet is almost as easy, and generally more anonymous. Therefore, threats on the internet are largely ignored (most are spur of the moment and in temporary anger). Like the parent said, if someone takes the time to write a letter (and attempt to disguise their address), then the situation is much more serious. The person clearly has a much more long-lasting anger and is more likely to deliver or attempt to deliver on the threat.

I think you meant: Hell, it's about time.