I agree with Larryjk- the niches for new cartridges these days are quite narrow. A lot of companies introduce things to have their own headstamp on them, on the theory that promoting their name will eventually lead to more sales of guns, ammo, or both. Hence you have Hornady introducing a 30 cal cartridge for lever guns that is a ballistic duplicate of the 307 Winchester. Slightly different case, but it doesn't do anything the older cartridge didn't- except say Hornady on it.
I don't mean to pick on Hornady- they all do it. The Remington SAUM line is another example.
On paper ballistics can sell some rifles- like the ultra mags and the STW's. These cartridges have a narrow niche, but I bet a lot of them get sold so someone can have the biggest/fastest/baddest. That's fine- but, after the buyer shoots them a few times, they get rid of them- too noisy, too much recoil, too expensive.
There are always some big bores being introduced, too, for the folks that think bigger is better. Most of those won't last, either. Some of the big monsters would be excellent on an African Safari, but how many people do you know that go on african safaris? I see a whole lot of the really big guns at gun shows, with a partial box of ammo, and no sign the bolt has even been worked much. A few shots at the bench and the buyer discovers his 270 wasn't so bad a rifle after all.
Woods cartridges in bolt actions have a long history in the US of not selling. I expect the 376 Steyr will go the way of the 358 Winchester- we all talked about what a good round the 358 was, but we didn't buy one (me included).
Firearms makers and ammo makers will introduce anything they think will sell, and keep it in their line as long as it is selling. When it doesn't sell, they'll drop it.
An example of a narrow niche for a new cartridge is the 450 Marlin. There may be a demand for that round for people that don't handload for their Marlin lever, but, time will tell if there is enough demand to keep that one afloat. A handloader can get the same performance from the much more common 45-70.
The truth is, there is very little need for any new cartridges.