Mr. Terrific had fallen out of sight by then, while Cyborg was popular from Teen Titans and had the somewhat unique gimmick of heavy robot tech, especially when they reworked him to be based on Apocalips tech instead of just being a general cyborg. Yeah he was younger than the rest but that might have been the point to diversify the ages a little by having a younger kid brother type to some of the others. Terrific was just another 30 something like most of the rest of the team and he just has his gimmick weapons. Of course Terrific got a recent boost from the last Superman film while Cyborg's fallen out of prominence so things might change.