I think the main reason is because this particular site (as opposed to others) is set up in a way to attract smart, thoughful people who really want to help other people, and to try to do that in a friendly, consciencious manner. If you stick around for awhile and get into some of the more contentious subjects, you will find out that nice-ness sometimes gets sucked out the window. But even then, other jellies will jump in and try to calm everybody down.
Also, the mods are here to make sure that no one attacks anybody and they swiftly remove anybody who acts like a troll, or a psycho or a big fat meany.
And the Fluther guidelines are pretty clear about how to post questions and answers in a reasonable (non accusatory, non threatening and non inflammatory) manner. If you try to do any of those naughty things, the mods will send you a private PM or they will send your question back to you, so that you can edit it into a more appropriate statement.
And most of the jellies on this site tend to do a lot of homework before they make a statement. Most (not all) jellies tend to edit themselves, before they post a statement instead of just shooting off their mouths. In real life, it’s hard to put the toothpaste back into the tube.
I hope you will stick around. We really are a nice bunch of joes. And we throw marvelous virtual parties!