Oh, they'll still try, because once in a while a new cartridge takes off and sells a bunch of rifles. But most new rounds lose money, because it takes considerable money to develop one, which means they've got to sell a lot of rifles to break even.
I must comment for the second time on this thread that most of the "new" centerfire cartridges mentioned on this thread are just tweaks of really old cartridges, especially the various 6.5's mentioned. Several that not many Americans even know existed appeared very early on in Europe, and even the .260 was a wildcat long before Remington and Jim Carmichel claimed to "develop" it. I believe Ken Waters made up his version, called the .263 Express, in the early 1960's.
Even the .300 WSM is just another version of wildcats going back to the 1950's, and all it does is reproduce the .300 H&H in a shorter case. Of course, some people still believe Winchester's PR claims that the .300 WSM's case shape results in ballistic magic allowing higher velocities with less powder than other .300's, but so far no pressure-testing has found that magic.
The main reason so many shooters believe many cartridges are "new" is most shooters don't know much about the history of cartridge development. I suppose even the .17 HMR could be considered a tweak of the 5mm Remington--but unlike Remington, Hornady didn't try to develop a rimfire cartridge to directly compete with Winchester's .22 Magnum.
The original factory load of the 5mm was a 38-grain bullet at a claimed 2100 fps, but it simply didn't exceed the original 40-grain load of the .22 Magnum by enough to make any difference, especially in public perception. But the .17 HMR's an entirely different deal ballistically, using much lighter bullets at much higher velocity--and is also consistently far more accurate than the .22 Magnum. Which is why it became so popular, so quickly.
In contrast, most of the new cartridges mentioned here don't do anything much older cartridges didn't do ballistically, except fit in shorter magazines. But that's because shorter actions became popular, requiring new cartridges to fit them. The basic ballistic performance isn't any different than a pile of other rounds that were developed long ago, some more than a century.
