One word: time.
From the Small Business Development Center at the University of California, Merced (link):
A well thought-out business plan generally takes anywhere from six months to a year to complete, but it can be less depending on how committed you are to the business and how much time you are willing to spend on writing your plan. (Emphasis ours)
Sierra Investment Group, Inc. is a small business plan writing service and small business funding resource. They can be reached toll-free at (800) 323-6298 from 8 a.m. to 4 p.m., Monday through Friday, or at firstname.lastname@example.org.