As part of in-depth interview with him in ONM 106, Phoenix Wright creator Shu Takumi revealed that after the character's third game, he had doubts about whether another should be made at all.
"I felt that Phoenix's story had been told, and that the series should not continue", the writer, whose most recent work is the fantastic Professor Layton vs. Phoenix Wright, explained to us. "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."
Of course, his next appearance was at the stand rather than his usual bench in Apollo Justice: Ace Attorney, a move that Takumi was initially against: "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."
The creator's previously expressed mixed feelings about Phoenix's return in the latest Ace Attorney (the first of the main series he didn't write), but after writing his new crossover, and committing to a new game in the series, it seems he's warming to the idea.