Why 10 years between colonoscopies? Can't something develope within that time, even if a good colonoscopy was done 10 years ago?

Statistics. It can take a long time for a precancerous polyp to grow and become a colon cancer. I think ten years was recommended because there are certain areas like indianna where we don't have the manpower to do colonoscopies on everyone every 5 years. If you have a family history or have had precancerous polyps, i would recommend sooner than ten years. These guidelines are based on probability.
CDC. Cdc guidelines also suggest 3 consecutive stool samples be checked for blood each year and a flexible sigmoidoscopy every 5 years. In general, colon cancer development is a long process so that 10 year window is usually adequate. Not everyone has an adequate colonoscopy, however, so it is important to utilize these back-up tests as well.