Sorry, I'm not getting what you're not getting. IRC channels and subreddits are defined around special interests. There is a thing that unifies people. Sometimes that topic can have a well-distributed membership, but there is always a selection bias that comes from the chosen topic bias.
Twitter is unique in that it had both wide international reach and no topic structure. Reddit is an engine for shutting you off from 99.9% of the Reddit discussion. Twitter, for better and for worse, didn't and couldn't do that.
As you say, the journalists were definitely part of that. To the extent a global conversation existed previously, it was among journalists and the people they covered, like major politicians, NGO heads, and the like. Twitter expanded and disintermediated that. Suddenly you could hear from all those media subjects directly. And even more unusually, you could try to join in, speaking directly to people previously unreachable, maybe even getting replies. One major motivator for people joining Twitter was exactly that, as the Waiting for Bieber art piece demonstrated: https://www.trendhunter.com/trends/waiting-for-bieber
The only platform with anything like that reach was email. And although email has global reach, it isn't a global stage like Twitter was. No other platform has that, and I doubt one ever will.