For one thing there are still quite a few people left over from when bungie made halo, they have made a community and would rather not leave it. On top of that these forums are now in app form, which makes them easier to access then most other forums, this makes them more commonly used by people who may not necessarily care for the current game bungie has out. Forums can be used by anyone, whether you like it or not, and the fact that the bungie forums are so easy to access just means you are more likely to get random people.