My Hero Academia and Jujutsu Kaisen both had amazing antagonists, each with the potential as one of the best ones in the Shonen genre. Before the deaths of Shigaraki and Sukuna in the final chapters of their manga, their plot armor was expected to keep them alive but it varied for both of them as Shigaraki's plot armor was much stronger than Sukuna's.
Before the final war, Shigaraki had various near-death experiences which he solely survived due to his plot armor. The prime example was the fusion between him and All for One which, despite not reaching completion, was somehow successful. On the other hand, Sukuna only had one near-death experience which was when he died, proving that plot armor was negligible in this case.
Disclaimer: This article contains potential spoilers from the My Hero Academia and Jujutsu Kaisen manga series and has the author's opinion.
Why Sukuna from Jujutsu Kaisen had a weaker plot armor than Shigaraki from My Hero Academia

Tomura Shigaraki was one of the main antagonists of My Hero Academia and Ryomen Sukuna was the final and main antagonist of Jujutsu Kaisen. Both of these antagonists were known for their sinister personalities that were obsessed with ending humanity (the former with heroes and the latter with sorcerers).
Both of them met their end in the final chapters of their series and died miserably which was expected from their sins. Tomura Shigaraki was taken over by All for One, the main antagonist, even after his death, but Izuku Midoriya freed him using his vestiges. All for One then died but Shigaraki couldn't be saved during the course of events.

Ryomen Sukuna couldn't keep his control over his host (Fushigoro Megumi), combined with an outside force that pulled him (from Itadori and Nobara). This ripped him off from Megumi's body, reducing him to a blob of meat. Despite Itadori's kind words to him during Sukuna's final moments, Sukuna couldn't comply with his kindness and claimed to return in the future.
As 'unsatisfying' as their demise could be in the eyes of the fandom, they were destined to die because that is given in any Shonen series. However, both of them didn't utilize their plot armor to the full extent which was obvious from the near-death experiences both of them had.

From the start of My Hero Academia, Tomura Shigaraki had only been saved from death by his plot armor as one of the main antagonists of the series. For instance, his battle against Gigantomachia, him being alive despite getting mentally paranoid due to All for On taking over him, and the finale where he was saved from All for One's control and was not nerfed as a side character.
However, Sukuna's case was different as he remained alive through his genius instead of his plot armor. Throughout the entire series, there was hardly a moment where Sukuna had a near-death experience. He even changed hosts by proper planning, meaning that Gege Akutami didn't rely on plot armor for Sukuna whereas Horikoshi did for Shigaraki.
Final Thoughts
Although it isn't particularly a bad or good thing for Sukuna to not utilize his plot armor in Jujutsu Kaisen, it is a given in the Shonen genre to protect the antagonist from calamities before the finale. This could be the reason why Ryomen Sukuna was different from the other Shonen antagonists.
Related Links
- My Hero Academia's Final arc's climax: Horikoshi's radical take on Shigaraki
- Why Shigaraki's death does ruin My Hero Academia's ending
- 10 Naruto characters who can decimate Shigaraki from My Hero Academia