Most likely first and foremost because it is NOT a discussion forum. It is a Q&A website. From the FAQ directly:
Avoid asking questions that are subjective, argumentative, or require extended discussion. This is not a discussion board, this is a place for questions that can be answered!
The Trilogy has a specific way of dealing with information, since it is first and foremost an attempt at being a canonical resource of information that constantly gets updated, for specific questions that do have real and exact answer.
The achieve this goal a few methods had been put in place to ensure that the above remain the main focus of the site. These are not barriers, they are a unique and different way of thinking and moving away from the old discussion forum concept of dealing with questions.
Part of this process is to gain trust from other members of the community in the form of reputation, and based on this more flexibility with regards to this, as members learn how the site and it's processes work. It is a simple and effective method, which once adopted makes any other forum or medium extremely hard to use.