Each good turn sets you up to swim efficiently on the lap that follows. That momentary loss of control and the extra energy you spend trying to set things right can quickly start a frustrating spiral of slower, more exhausting laps, leading to more poor turns. The elements of a great flip include speed, ease and momentum.
Speed is obvious.
If each turn is hard work, they become progressively more ragged which sets off the spiral mentioned above and your stroke falls apart as quickly as your turns. Throw a rubber ball against a wall. Throw it slowly and it comes back slowly. Throw it fast and it comes off fast. Great turn mechanics maintain momentum through the somersault and off the wall.