I remember at the time his loan was announced one or two sources said he'd already signed his contract so it was a matter of activating the buy option. If I were to guess, the additional waiting period is to wait out some clause that Juve inserted in the deal to take him to Atalanta, so that Atalanta keep a bigger percentage of the feeAs long as it’s all sealed and signed, I’d hate to see the likes of Chelsea or City come in and steal him.
