Nested threading for long-running threads only really makes sense when you can manage the “read” status per post (as in Usenet and mailing lists). Otherwise, new “leaves” or subthreads in the thread tree are all over the place, and you don’t know which you’ve already seen/read and which you haven’t. Web forums generally only track a single “water mark” per thread for how far you’ve already read, which more or less implies that only chronological order makes sense. You still have hyperlinked indications of which post is replying to which other post, via quotes.