Actually, the technology was already present and ready when Hammond released the original SKs but Nord had the leap on them in terms of the vast amount of work and investment they'd put in to their piano and sample libraries.
In retrospect, the extra voices feature was a bit of a half-assed attempt to move in on Nord's territory, and in the intervening years, they've spent almost zero time and investment expanding or improving the library (reed organ, handbell, anybody?). During the same period, Nord has released several new grand pianos alone.
It could be, of course, that the amount of memory in the SKs just wasn't sufficient for decent size samples I'm not sure how much memory there is on an SK but I'm guessing the fact that Hammond never mentions it means it's negligible compared with its Scandinavian counterpart.
Of course, it could be that the reason Hammond hasn't bothered much with the current SK library is that it realised its limitations and decided to work on a new library for a more capacious successor (the SKX?). Let's hope so.
I can't remember the amount, but I remember having to delete librararies to install others, and they were not that large.