No, it's because Roark is too awesome to hang out with raw fail like Dawn. Also, he actually has a city to run, unlike Brock.
Brock stuck around for as long as he have because the Japanese fans loved him - the same reason he war brought back after his disappearance during the Orange Islands. Frankly, though, Brock has been pretty much 'plot exposition guy' for like forever - and I'm pretty sure that if it wasn't for the whole reboot air Unova got, he'd have stuck around for Unova too.
Also, the Orange Islands have always been an anime-exclusive filler arc. That's what they were meant to be, pretty much. As it was a filler arc, they had more leeway with the gym leaders etc. Working the modified gym battle format (because they were rarely actual -battles-) into the games would be... difficult, at best. Gimmicky, certainly. And do we really need more gimmicky side-games?