A linear thread is a good accurate model of a single conversation. Tree-structured threading is a half-solution because it doesn't support re-merging discussions and only tends to fracture them. Even when people aren't explicitly replying to multiple posts, each new post is implicitly a reply to everything that has come before, i.e. the current state of the conversation.
In the glory days of newsgroups, every reader ran their own news reader client software to keep track of what they'd already read and what was new to them. Without that it is very hard to follow ongoing tree-threaded discussions. If web forums attempted the same, that burden would be pushed onto the server.
A linear thread is a good accurate model of a single conversation. Tree-structured threading is a half-solution because it doesn't support re-merging discussions and only tends to fracture them. Even when people aren't explicitly replying to multiple posts, each new post is implicitly a reply to everything that has come before, i.e. the current state of the conversation.
In the glory days of newsgroups, every reader ran their own news reader client software to keep track of what they'd already read and what was new to them. Without that it is very hard to follow ongoing tree-threaded discussions. If web forums attempted the same, that burden would be pushed onto the server.