The Teenage Mutant Ninja stated two reasons why Raphael never outlived his brother and become the Last Ronin. Here is a quick glimpse of them.
Not Raphael But Michelangelo Is The Last Hero
In a dark vision of the TMNT’s future, there only remains one out of the four brothers who manages to survive. However, the dark vision revealed why Raphael could never make it to become the Last Ronin. In Teenage Mutant Ninja Turtles: The Last Ronin by Peter Laird, Kevin Eastman, Tom Waltz, Isaac Escorza, and Esau, it is shown that how each of the turtles and their flaws led them to a downfall. The story aligned right with the TMNT lore because the revelation of the survival was supported by the current actions and past of the brothers.
As soon as the announcement of the series was made, fans had their speculation over which brother is going to take the revenge of other’s death by Shredder’s grandson. The signature color masks fueled the mystery that was used to differentiate them and their signature weapons. While many theories stated Raphael as the Last Hero, it showed up to be Michelangelo who carried this role.
Michelangelo Is Calm While Raphael Is Hot-Headed
Even after the Last Hero got revealed, theories about why certain turtles never continued their role kept showing up. As far as Raphael’s character is concerned, it was his rash decision-making and hot-headedness that constrained him from becoming the last turtle standing out of all. These traits were specifically showcased in Teenage Mutant Ninja Turtles #121 by Jodi Nishijima and Sophie Campbell. In the plot, Raphael pursued an Old Hob to stab him while it was Michelangelo who pursued cold and calm-headedness to find the key to survival.
Thus, Raphael’s characteristics highlighted his susceptibility to anger, impulsiveness, and desire to lead every decision himself for the team. With such numerous flaws, the creators knew where to destine Raphael’s character for the Last Ronin.
With that said, Raphael’s struggle to control his anger and his desire to handle everything on his own led him towards self-destructiveness and not becoming the Last Ronin.
Source: Screen Rant