Because people use it to create artificial drama, such as, "Nobody likes me here, I'm leaving." Then a bunch of people respond with, "No, we like you! Please stay!" and then, "Okay, I'll stay."

Having done online communities for over 21 years, I've seen this happen more often than I can count. Since we like to be drama-free here in the community as much as possible, we remove possible mechanisms that reinforce such behavior (whether drama is intentional or not).

If you want to leave the community, you just go. PM your friends to let them know, and let that be it...

Best,
DocJohn