If you go back 10 yrs or more you will find a lot of modules available, and they were used to add (Expand) voices, styles, effects etc of the instrument you were playing, because with the technology of the time a large number of sounds etc could not be incorporated in a single instrument, however with the latest instruments this is no longer the case and so sales of modules would be small.
Another point to note is that better quality softsynths (VSTs) are available at a much cheaper price then a hardware equivalent, (VSTs also tend to make most hardware modules look primitive) and this combined with the fact that a computer (Or Open Keyboard) can have many softsynths available with only a small increase in weight, means that this is the way most people go. (Multiple modules (Expanders) are a pain to set up and need a lot of room, as well as weighing a ton, and this means that only those with roadies would be likely to use them)
Such is the advancement of technology

Bill
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English Riviera:
Live entertainment, Real Ale, Great Scenery, Great Beaches, why would anyone want to live anywhere else (I�m definitely staying put).