Because three single-core cables each throw off a magnetic field, and how you arrange them decides whether those fields fight or cancel. Clip the three phases together in a symmetrical triangle, which is what trefoil means, and the fields very nearly cancel outside the group. That keeps induced currents and stray heating low in cable glands, armour and nearby steelwork, and it keeps the three phase impedances equal so the load shares evenly.
Lay them flat and touching instead and the field is unbalanced and the impedances unequal. Spaced-flat is sometimes used deliberately to get more air and better cooling around each cable on a heavily loaded tray, but then you re-check the current rating. Either way, cleat single-cores firmly, because a short-circuit throws them apart hard.