Probably because the number of people who can read English greatly outnumbers the number of people who can read Japanese? It's very common for Japanese to use

Probably because the number of people who can read English greatly outnumbers the number of people who can read Japanese? It's very common for Japanese to use English labeled slides and diagrams for presentations that they know they will be taking abroad because it is much easier change the language of the explanatory text than it is to edit images to make them English.