Quoting from the KatoUSA website (the same applies for Japanese stock):
"Generally speaking, parts for a specific model are supplied only at the time of model production. Parts are then restocked only at the time of a subsequent production of the same model or if the identical part is utilized on a different model. Consequently, because of the wide range of models produced and new models continually being introduced, some parts may become unavailable between productions. In addition, because we consistently look for ways to upgrade our models, earlier “versions” of certain parts may not be available after being superceded by new components."
Unitrack, Unitram and buildings.
Locomotives and Rolling Stock, North American and Japanese outline
"Probably the largest range of Kato Unitrack, Unitram
and buildings available in the UK today"
Train Trax exclusively Kato!