请问steamer trunk为什么是皮箱？

这是steamer trunk 的英文介绍：
steamer trunk 是指一种上面平盖的箱子。它们是古董木箱。
Steamer trunks which are sometimes referred to as Flat-Tops, first appeared in the late 1870s, although the greater bulk of them date from the 1880-1920 period. They are distinguished by either their flat or slightly-curved tops and were usually covered in canvas, leather or patterned paper. There has been much debate and discourse on what these types of trunks are actually called. In some old catalogues, these trunks were called "packers", and the "steamer" trunk was actually referred to a trunk that is often called a cabin trunk. An orthodox name for this type of trunk would be a "packer" trunk, but since it has been widely called a steamer for so long, it is now a hallmark of this style.