Despite the latter being the king of curses, Itadori can suppress him because Sukuna doesn't have all of his power.

Source: https://www.cbr.com/jujutsu-kaisen-theory-suguru-alive-inside-possessed-body/

Hopefully, before ends, he can finally explain who exactly Yuji Itadori is and how he was able to suppress Sukuna, the most powerful curse ever.

Source: https://screenrant.com/jujutsu-kaisen-yuji-resist-sukuna-geto-kenjaku-choke/

Junpei Yoshino is the tragic character who teaches Yuji Itadori the depths of evil that Curses are willing to go.

Source: https://www.cbr.com/jujutsu-kaisen-main-villain-power-level-ranked/

Megumi’s biggest challenges was to confront Sukuna after the antagonist swapped bodies with Itadori.

Source: https://gamerant.com/jujutsu-kaisen-best-quotes-by-megumi-fushiguro/

Sukuna could never have guessed that Itadori would be able to suppress, and at times even control, his spirit, but the power is still unpredictable.

Source: https://www.cbr.com/jujutsu-kaisen-where-to-watch/