1) It's much more conveniant and saves time if one can just go to one thread to find out all the information on the topic- rather than hopping through 3 Space Mt. threads, some of which may overlap, one can just go to the single Space Mt. thread and find out all the information there.
2) It may be just me, but I tend to only look at threads on the first page (newest). For evrey new thread that is created, a pre-existing thread is pushed to the second page, where I believe it is viewed by less people. So if you create a thread, and would like many responses, you don't want your thread to get pushed to the second page, especially if you have not yet gotten an answer.
Others may have better reasons, but these are the ones I thought of.