The CC upper is not open at the bottom because the Eagle launcher is rail mounted.
Colt USA monolithic uppers have removeable bottom pieces because mounting M203 was still a thing when they were conceived. They are made to be legacy M203 compatible, all the way back to the SCAR development days. Since M203 is replaced by HK302, open bottom system is a thing of the past.
The newest M901 doesn't even have a monolithic upper anymore. They just pin an extrusion tube handguard to the upper, much cheaper and faster to make.