Ever run a manufacturing business? Perhaps it's because:
Norma makes it - and they'd be competing for that business. There are a lot of 300 wsm shooters but are there enough to satisfy the sales requirements to make that kind of move cost effective? I've never seen Normal 300 wsm brass on backorder.
It costs big bucks to tool up for a specific caliber. Would their return on investment (given the Norma competition) make it a wise business decision?
They already feature more than two dozen types of brass and when you're in business you can't fill every niche in the market.