Anon:
Unfortunately, I cannot answer the "why" question you have. The ITTOs that are called on the PMBOK Guide (remember, it's only a guide, not religious scripture) are there presumably the authors of the guide thought those are the ones that are used most often or are important enough to highlight.
From a non-PMI or non-PMBOK-y way of looking at things, If I were to do a project (call it project X) and I had a repository of historical of information on projects similar to Project X, I would like to know about both how much they cost (budgeting) and how long they took (scheduling).
Hope that helps,
Harry