Often in bonsai it is hard to tell or confirm a tree species as some of the characteristics of the tree change quite considerably when maintained as bonsai. Leaf size for example may be many times smaller than it would usually be on a wild specimen of the same species of tree. One way to ensure we are all talking about the same species of trees is to use scientific names but more often than not there is a tendency to shy away from scientific jargon for use of common names.
Common names are easier to write, often shorter, pronunciation is easier, there can be localised understanding through use of the common name as well as a colloquial edge. However on the down side it can lead to misinterpretation and incorrect identification of a tree as some trees have multiple common names like English Oak, Common Oak and just Oak.
By using scientific names it provides a greater level of understanding and professionalism and is widely recognised throughout the world so an English Oak would be Quercus robur.