Because this thread has essentially fulfilled its purpose (to hold 150 images) and has nothing else to offer that cannot be done in the new thread (which is also more active). It is simply a matter of education: the "full" threads are no longer pumped into page one so that other threads that have not yet reached their limit of images take their place on that page and have a better chance of receiving attention. In addition, posting in the old thread fractures the conversation and reduces the chances that others will see your comments (I for example only realized what you did because I still have this thread stored in the thread monitor). If you want to comment on images in old threads you can do it in the new threads without problems, because 4chan will automatically link the post to which you make reference.
The only reason you can bump "full" threads is because the developer is a complete idiot who refuses to match the limit of posts and files to prevent cases like this.