The post form is kind of a mess. Too many things going on in there. Also, the part with the checkboxes breaks horribly if the browser window is narrow. Typing things into the normal textboxes may be unintuitive to new users, but it also doesn't overwhelm them with mysterious options they don't understand.
Also, forking seems like the kind of thing that just adds complexity and confusion. One of the key features of this kind of board is the simplicity. You don't need to have an automated option for everything, if you can do it by hand just as easily. Like forking, people could just say "Hey I am starting a new thread on the topic brought up in post so-and-so", and that will be much easier to understand for everyone involved.
With a lack of complex features, people use natural language to describe what they are trying to do instead, and that tends to make things easier for everyone involved.