One thing everyone has to realize is that arranger manufacturers (and keyboard makers in general), have primarily one thing as their first concern and that is $$$.

Having a modular based system where you can get the styles you want on the same arranger would not bring them as much money as making you buy a new hardware board.

Take for example the Roland GW8. They have the latten version (hardware), the East European version (hardware) and the Asian version (hardware).
Why didn’t they make one master keyboard and have the ethnic sounds and styles as plugins?

Also look at some of the other brands that have oriental versions (hardware) of their arrangers.

It is better for the arranger manufacturers to sell a hardware unit than just and additional sound and style set.