Mountains and hills are made from different goological material. Try to review different rock types (metamorphic, igneous and sedimentary). Depending on what materials a mountain or hill is made of will determine how long the material can resist erosion from weathering factors. Also, hills and mountains have been created at different times, so this issue can also influence whether they are talloer than other peaks. In the U.S., the Appalachians Mountains are much older thant the Rocky Mountains. The Appalachian Mts have experieced a lot more weathering than the Rockies, and are not as tall.
