"I felt that Phoenix's story had been told, and that the series should not continue", Takumi told Official Nintendo Magazine in a recent interview. "Knowing when to end a story is very important and I wanted to avoid dragging it out and having it become a shadow of its former self."
Trials and Tribulations provided a nice cap on the story of Phoenix Wright, a young lawyer with a penchant for bluffing his way into proving clients' innocence, that started on the Game Boy Advance in 2001. That's why when Capcom decided it would push the series forward with or without Takumi, the designer came up with a new ace attorney. "Years later, when it was decided within Capcom that Ace Attorney 4 would go ahead, my position was that we should change the main character and tell a new story. I didn't plan to have Phoenix in the game, but my colleagues said they wanted him to appear in some form, which is why he ended up being the accused in the first case in Apollo Justice: Ace Attorney."
Two Ace Attorney Investigations games and a fifth Phoenix Wright game, Ace Attorney: Dual Destinies were produced without Takumi. He didn't completely walk away from the franchise after Apollo Justice; he wrote the plot for Level-5's Professor Layton vs. Phoenix Wright: Ace Attorney. He still hasn't directed a mainline Ace Attorney game since 2007.
That will change in the near future, though. While Takumi once wanted to walk away from turnabout tales, he's already working on a brand new game in the series. The new Ace game, with Takumi at the helm, was announced during a Nintendo Direct event in February.
Phoenix Wright: Ace Attorney - Dual Destinies (Turnabout Reclaimed)