they do compete... in the OQ... if I'm buying a 6.5mm barrel for my AR, I'm likely to only buy one, or even if I buy more than one, they are likely to be of the same caliber to save on loading dies, etc... ( unless you're a dumb donkey like me, who gets a kick out of loading tons of different cartridges )
but to go back to the 17's I mentioned... the 17 market consists of 5-6 major players ranging from 17 Mach 2, 17 HMR, 17 Hornet, 17 Fireball, to 17 Remington... not too many regular shooters are going to buy more than one 17 caliber rifle... if you are not a reloader / like rim fire cartridges, the Mach 2 & HMR are your choices... Hornady really put the initial push out with the HMR, & now finding a Mach 2 is pretty tough ( I have a custom rifle I had built in 22 Mach 2 that I love, & shoot regularly out to 150 yards ) ( the Mach 2 is based on the necked down 22 LR cartridge, the HMR, is based on the 22 rim fire magnum case ) being both are rimfire, buying loaded ammo is definately the easiest way to go... so I always keep a couple bricks of Mach 2 on hand, before they disappear completely...
... in center fire, the 17 Hornet, & Fireball were wildcats, that were reciently "standardized" Remington came out with the Fireball 1st, but spent next to nothing advertising their "new" cartridge, & it has all but disappeared... Hornady has been advertising the 17 Hornet, offers factory ammo at little over empty case price, & though in my opinion, is no were near as balanced a cartridge as the Fireball, or as easy to reload, is likely to find more of a market share, because of the effort put forth by Hornady... & likely should become more popular than the HMR, for those that are reloading...
the old 17 Remington ( a 223 necked down to 17 ) was never my cup o tea... my FIL shot it a lot, & had loads that would vaporize the bullets before they'd get to the target... but the cartridge IMO, uses too much powder, burns barrels out, & seems to be pretty much gone, except for a few die hard enthusiasts... ( BTW... my custom 17 Fireball was built on a 60's Remington 700 that was my FIL's that had enough throat errosion, that it would no longer group... rather than chambering it back in the old 223 based 17 Remington, I chose to rebarrel & shorten the mag, & chamber it in 17 Fireball )
point being, I'm a bit not normal, & chose one slower, one hotter 17 ( yes, I do have that odd 17 K Hornet Contender barrel, that the new Hornady 17's fire form in, & an old 17 Remington Contender barrel, that used to belong to my FIL )... IMO, the 17 Fireball is the best of the group, easiest to reload, & most balanced, yet seems to be the 1st to die, not because of the cartridges merit, but because of the lack of support Remington offered, & the market share they got from the cartridge with the little effort they put into it...
I'm sure several of these AR cartridges will suffer the same fate... may be the best at something, but too much competition, & lack of market share for what ever reason will kill a couple of them for sure... though there will will always be dumb donkeys like me loading them, long after they've gone obscure