Mainly because I could for much the same price as 6-inchers.
Atmos speakers are for effect more than substance but 8-inchers will probably move more air.
I would look for units that have concentric but pivoting tweeters, are well built, easy to fit and secure once fitted, and are designed to cope with insulation and dust build-up.
Technically you should box in your In-Ceilings as this helps retain the fire rating of your ceiling, will stop insulation clogging up the speakers, stop the wee beasties eating them, and ensure there is always free air around the crossovers to stop then overheating. Boxes also push the sound down into the room, rather than letting it escape into the void. My media room has an attic above, so I simply knocked up some plywood boxes (no prizes for my woodwork) and glued them over the holes, on top the plasterboard. If you don't have that luxury you can buy ready-made steel or plastic cans on Amazon, eBay, etc., but they can be unnecessarily expensive.
The only criticism I have of my Yamahas is that they use sprung bare wire terminals, not screw-downs, although they will also accept banana plugs.