from a manufacturing standpoint, its a massive pain in the ass. it requires a non standard bullet diameter that in most product lines it is the only loading requiring it, same with the case head, a rimless .473 drawn brass blank can be made made into over two dozen different cases at least, but the .303 rim is unique to itself. and with no mass production of new guns, far lower demand.
most manufactures will only do a run of oddballs like .303, 30-40 krag 38-55 once a year. and if market or military demands more mainstream offerings the oddities are the first cut from the schedule.