With all the urban combat, there's a push toward shorter and maneuverable. In something short like the 10.5" M4A1 CQBR, flash becomes a much bigger problem (especially in dark rooms where it can be positively blinding to the shooter).

A suppressor helps with this issue, but it also starts adding back length to the gun (somewhat defeating the purpose).

300 blackout supposedly helps the problem (I've never shot it from a SBR) and also has better short-range (esp subsonic) ballistics compared to 5.56, but it too has muzzle flash.

The newly selected Army XM5 rifle and cartridge were designed for super-high pressures and achieving high speeds in a short barrel likely causing even more muzzle flash. Thankfully, this rifle is going to be suppressed for everyone by default. Unfortunately, it's super heavy (and that's without the super-heavy optics), has less ammo per magazine, and despite the goal, is still going to be blocked by higher-level body armor, and is still 36" in a SBR configuration (for comparison, the Israeli CTAR-21 is 30% shorter at just 25" providing rifle firepower in a SMG package).