For one thing, there is not as much money to be made as introducing a new caliber in a new rifle. My book "Cartridges of the World" is almost two inches thick and the typical page has two cartridges, sometimes three cartridges, rarely one cartridge. Cartridges are introduced not to fill a "gap" as to increase the profits of the cartridge manufacturers, gun makers, die makers, etc. The whole ecosystem depends on growth and expanding the market.
Someone here would know if this is called elastic marketing, line filling, or downward stretching. I have been looking for the term that is used for increasing market demand through insignificant changes.
Others have noted that the 30 cal carbine was not a particularly powerful round and it gained that reputation in WW2 and Korea. The real popularity of the M1 Carbine was because it was light, and anyone who had to walk up the hot, humid, hills of New Guinea carrying 60 to 70 lbs of gear, and a rifle, loved the lighter weight of the carbine. It gained a particular bad reputation in Korea against Chinese wave attacks. GI's thought that the bullet was not penetrating due to ice on Chinese uniforms, but that was doubtful. I asked a Vietnam Veteran and retired Colonel, whose Dad (retired Colonel) was a WW2 veteran and was in Korea as the Chinese pushed down from the Chosen Reservoir. He went through Chinese wave attackers in minus 40 F weather. Dad's reply was "
the Chinese were all hopped up, they were all hopped up!". On clarification, hopped up meant drunk or some other altered state. While it is very possible that the Chinese soldiers had drunk themselves to a stupor the night before their suicide attacks, the fact of the matter was, they were brave men who were not going quit until they had been physically broken down by enemy fire. And the 30 carbine round was not up to the task. It was simply a pistol round replacement, that unfortunately was used as a front line battle rifle because the package was so lightweight.
Based on that , many of the hunting laws that went into effect in the 1950's and 1960's specifically excluded the 30 Carbine precisely because Veterans making the rules knew that the cartridge was not humane. I am certain the M1 carbine was not cheap to make as the receiver takes a lot of milling.
National Ordnance made cast versions of the things, and looking at the linked web site, it is surprising how many post war firms made the things. But the aftermarket carbines I saw, were generally made on the cheap, not something to inspire confidence in the things.
Even though it was not until 1973 that the mini 14 was introduced, who would really want a light weight rifle in 30 Carbine when you could buy a similar design but in the front line cartridge used by the Armed Services?