Upload & Sell: On
Instead of the zoom, or in addition to? I would bet a couple of my paychecks they simply feel a zoom is much more marketable to the mass of consumers. I'm sure they've looked at the popularity of the Canon 100-400 zoom and the Sigma 150-500. However if they did a prime ALSO, I would certainly be a potential customer if it's nice & sharp.
Yes, the long zooms are certainly more marketable. That's why there's a handful of them available at this time. But there is a smaller market for a slow but sharp prime. And right now, there aren't any 500mm 5.6 or 600mm (6.3 or f8) lenses available. The market for such a lens is wide open if someone wants to step up and manufacture one.