What makes detecting a 'cold' Jupiter orbiting far out difficult using the transit method?

Its transit window occurs only once every few decades or centuries

The difficulty in detecting a 'cold' Jupiter—a planet orbiting at a great distance from its star, analogous to Neptune's position in our Solar System—stems directly from the mechanics of the transit method. Because these planets travel on very long orbital paths, the interval between successive transits, relative to an observer on Earth, can span many decades or even centuries. The transit method requires repetition for confirmation and reliable measurement. Therefore, the necessary observation baseline required to catch one of these rare alignment events makes these distant worlds significantly harder to confirm compared to their short-period counterparts.