One obvious reason is, it's easiest. I hardly know anyone's private contact info, but of course you can always look up colleagues' work emails in the corporate

One obvious reason is, it's easiest. I hardly know anyone's private contact info, but of course you can always look up colleagues' work emails in the corporate directory. And a large part of organizing is bringing people into the fold, so you'd still need to be organizing at work somehow, even if all you're doing is asking for external communications methods to continue the conversation. There's a much higher drop rate that way though.