Because being online means that there is distance, anonymity, and removal of blame. You can't see the person's face, or hear their voice, or see their body language. You can't always tell exactly how to intepret their words and what tone they're using.
I, personally, think that people on Bloop need to relax and slow down. People on here get riled up over the smallest things sooo easily. I read the various back-and-forth with incredulity and amusement. And I'm sure it's not just Bloop - I think it's evident in forums in many places.
My argument isn't, of course, that it's "just the internet" so people should "get over it" and "chill". People do tend to be more rude on the internet than they do face-to-face, for the reasons that I mentioned above. People instead need to be mindful of the fact that words are very powerful, but to not let them get to them on an online forum/site. The good thing about the internet is that it's much easier to close the browser and walk away without leaving an angry, scathing comment than it is to just turn around and walk away from a person when they are talking to you face-to-face.
People should practise this art of zen. However, many will continue to ignore it, as they thrive on the drama. xD Live and let live!