So why is the 5.56mm being replaced with the 6.8
A variety of reasons that I will list in no order of prominence (keep in mind that some of these reasons may not be entirely accurate/conspiracy theories and or not an explicit reason for the NGSW program)
1, Army got butthurt about Talib's shooting at them from 800m away with PKM's in Afghanistan and having to rely on JDAM's to kill em'
2. 5.56 out of short barrels has lethality issues, this has been established fact since Operation Gothic Serpent.
3. M855 sucks balls on flesh compared to M193, especially out of short barrels
4. M855A1 is a hell of a lot more expensive than M855 for similar exterior ballistics and can't pierce armor
5. Chinks are starting to have decent Body Armor and Russians have had decent body armor for a while now
6. The American public have been buying armor like crazy since 2020 and the ZOG Globohomo NWO wants to be able to kill Bubba (Or something like that depending on what sources you trust)
7. Last war syndrome, similar to reason 1 but about long distance shootouts in Mountains
8. Sig bribed some Generals
9. Generals wanted LSAT 2.0 but where legally obligated to host a competition with other participants
Now if the forum post @AGPinochet shared is to be believed, the idiots in charge might have realized that asking for a higher powered cartridge in a lightweight gun means there's going to be a lot more wear on the barrel and are looking into the 6.5 instead.
Not just wear and tear concerns, you have to remember that the military is increasingly comprised of 5'7 mexicans and women, The test firing videos I've seen have NGSW guns knocking around big burly 10th mountain division guys.
Though, from what I understand the plan for the NGSW program is to only roll out the new systems to the special forces and recon in 2022.
I don't remember where I read/heard this but I believe I either heard/read that there was a plan to only outfit around 20% of the Ground forces with NGSW guns and form them into super duper mega QRF (more accurately described as an Expeditionary force maybe) that can respond really rapidly to say, a Chinese invasion of Taiwan or a Russian invasion of Eastern Ukraine/The Baltics.