Rafts can do the miles - but they need to be on the river for more hours a day than kayaks in general to do it. It's 225 miles to Diamond, over ten days that's 22.5 miles a day. The current moves at around 4 mph on average (depends on flow, where you are in the canyon, etc). So about six hours of actual downstream travel gets it done. Add in some reasonable rowing and you go about one mph faster. Kayaks can add more like 2-3 mph with reasonable effort - so it's hard to keep a trip together if the kayakers are pressing the pace. If a raft goes and everyone adds gear to the raft - it's going to get heavy quick - heavy boats don't move much faster than the current.
Often the travel time issue starts when you leave camp in the morning. Most river trips I've been on leave camp at around 11 am. Stop for lunch around 1, get back on the river at 2, then stop at camp around 4 or 5. So 4 to 5 hours of downstream travel per day. That's about 16 to 20 miles a day - unless you stop to hike, or scout then it's less - often significantly less. If you can get on the river by 10 am, take one hour for lunch and don't mind getting to camp at 6 pm, you get 7 hours and about 28 miles covered assuming no hiking or scouts.