EDIT: Look at the avarage amount of vehicles for each nation. Japan has a total of 69 tanks (including event and premium ones), compared to the 161 for USSR, 128 for the USA, 151 for Germany and 116 for Great Britain. Despite Japan being added two and half years later then Germany and USSR, 1 year after GB and 2 years after the US.
Which means over the total amount of the existence of Japan (and all other nations after the addition of GB), have been getting less tanks per update then the main 3-4 nations ingame.
Which is on purpose and a real shame, since most/if all nations still have so many interesting vehicles left - which can be added in the current game.