This is answered by Geto near the start of episode 14 "Kyoto Sister School Exchange Event—Group Battle 0—" (Transcription: "Kyōto Shimai-kō Kōryū-kai—Dantai-sen ⓪—"; Japanese: 京都姉妹校交流会—団体戦⓪—). Basically, a Binding Vow needs to be initiated of one's own volition. Externally pressuring someone into a pact does not count, and tricking someone into accepting a Binding Vow is extremely difficult. So, it would be a waste of effort to try and force Itadori into a Binding Vow with Sukuna by taking a hostage.
This is the conversation between Mahito and Geto in episode 14:
Mahito: The game started when I met my new toy, but it didn't go well. It wasn't bad at first, though. I should have taken the toy hostage, and forced Itadori to make a Binding Vow.
Mahito (speaking to Itadori in his imagination, while holding Junpei hostage): If you don't want him to die, hand over your body to Sukuna.
Geto: No, a Binding Vow can only be done by himself. It's not that simple to initiate a Binding Vow through intervention.
It is interesting to compare this with how Sukuna manages to get Itadori to accept a Binding Vow as per Sukuna's conditions in episode 6 "After Rain"
(Transcription: "Ugo"; Japanese: 雨後). Sukuna obviously understands that he needs Itadori to agree to it of his own free will, and he manages to get Itadori to do so by adding a small additional condition: if Itadori defeats him in a battle, then Sukuna will revive him unconditionally; else Sukuna will revive him as per the conditions of his Binding Vow. Goof that he is, Itadori promptly agrees to this, and is just as promptly defeated.