You also have to consider that the preferred range of sounds are different for arranger and synthesizer players and the degree of editing. Some arranger players would quickly complain about the confusing depth of editing options from filters and amplitude envelopes to the waveform level. The way instruments are sampled is also often adapted to certain tastes (I'm thinking of saxophones with a certain delayed vibrato, which sound like from a ballroom swing band, vs.dry pop saxophones). So they would have to throw both sound pools in, which means double storage capacity. Inevitably, both arranger players and synthesizer players would leave a big part of the facilities unused, so they would have to buy a sort of oversized package. It contradicts the idea of market segmentation into distinct customer groups with different needs, and, as the others already said, the group who need both features is too small to be relevant for sales.