Z on a mini-run, 24-20, this Q...

I don't understand why coaches are so rigid with their rotations. Beans led by 22 with their second unit in. Then I saw PGJR-13 check back in. Why? I know he's having a great game. WHY? Why not let the second unit go until the other team starts coming back on you. You put him and the others back in at the peak, they're cold.