Might have been a cost measure, although I can't imagine there being a significant difference between a hydraulic actuator and an electric one. Maybe it being in a water environment and there being less chance for a short in a hydraulic system versus an electric one? Or maybe animitronics just use hydraulic where a ride vehicle would use electric? Something about the difference in weight maybe? As in an animitronic would not need something as strong as a ride vehicle? Who knows.
^ which leads me to wonder, if they can make the scoops move quickly like monster beetles with linear actuators, why can't the use actuators on all animated figures... at least the big ones? Everyone ponder that and get back to me.