Beech has been used because it is the premier bench wood. It is reasonably hard and heavy and it has the ability to absorb shock and vibration, which is nice it you are working on it all day. Much better than hard maple.
Andre Roubo (1769) says that benches are commonly made of beech and elm (both of which absorb shock), but that beech is preferred and most common because it has a finer grain. It is kind of annoying to write on a piece of paper on a bench made of ash or oak or elm because of the pores in those woods.