NOT being a concert pianist myself, I can only guess, but, according to my teacher, who WAS, they maintain a "short list" of pieces that they program around for one or two seasons. They get booked for recitals a couple of years in advance, especially for concerto work, so there is plenty of time to prepare new stuff. But once they have a concerto on the "books" and learn it, their agents go to work getting more bookings for the same concerto. at least that's how my teacher describes it.