Missed it by "that much"....
While Remington has had a number of smashing commercial successes since WWII, they have also produced a number of quite good cartridges that failed to catch on with the public, once the newness wore off.
Some of these are now essentially dead, and the rest are generally living in retirement homes, along with the majority of us who bought them.
As usual, Wiki gets it mostly right. But leaves out details that actually explain important matters.
While both the .243Win and .244 Rem were introduced in 1955, I believe the Winchester round got to the market first. American gunwriters, most notably Warren Page touted the .243/6mm bore size (and the .243 Win) as the ideal dual purpose round, varmints and deer in one package. And, it is a good round for that.
Winchester rifles were given a twist rate that would work well with both the light varmint bullets AND the 100/105gr "deer" bullets.
Remington envisioned the round as a varmint caliber (seriously mis-reading the market), and built varmint rifles, giving them a twist rate excellent for the lighter varmint bullets, but marginal for the longer, heavier deer bullets.
When the word got around that the .244s wouldn'd group worth spit, with deer loads, the buying public, who had been taught they wanted a dual purpose round (the .243) took a pass on the .244. Sales (never big) slumped.
Later, Remington changed the twist rate of the .244, but it never regained any popularity, the "word" was out about it, and buyers still mostly passed.
(I have heard conflicting versions about the change in the twist, some say Remington did it after a couple years production, and others say it took Rem 6 years to get around to it, changing the twist only a couple years before ending production of the .244 in favor of the 6mm.
The 6mm Remington IS the .244. Cases are the same, except for the headstamp. Rifles are the same, except for the markings and the twist rate (and there are .244s with the faster twist).
Handloaders know they can get about 100fps more from the larger .244/6mm case than they can from the .243 Win. This makes a noticeable difference shooting varmints, with rifles built for the purpose. Put both .243 & .244/6mm in a carbine size deer rifle and the difference becomes mostly moot.
The .243 was the gunwriter's darling, got an early lead in sales, took a huge jump when it turned out the .244s shot poorly with deer ammo, and stayed there, ever since. Remington renaming the round 6mm (1963) has kept it alive, but it never has done as well as other calibers. 6mm fans are loyal, and so there are still 6mm rifles and ammo, but not many, while everybody and their Uncle Max builds and sells a .243 Win.