I would say the reason this fragmentation never happened is cause from day 0 we actively discouraged it.
Sure, random person could create a subreddit for Discourse but we (Discourse co workers) would never visit it so the motivation from the community to fork the community is super low.
Same goes for chat, over the years there was an attempt or two by the community to make a fast lane chat place, but as Discourse co workers never really visited it. It fizzled.
The combination of allowing for healthy discussion here plus simply … being here and showing progress here, meant that no new venues were required.