The term ‘visual novel’ is a rather poorly defined one, most definitions don’t go into much more detail than ‘something combining words and visuals’. To add to the confusion, the Japanese and English usage are very different. I’ll focus on the English meaning of the term.
Visual novels differ from the adventure game genre that brought them forth in the balance between story and gameplay. Most adventure games are heavy on plot, but wrapped around core gameplay consisting of puzzle solving and exploration. Visual novels on the other hand contain little or no gameplay. Narrative with some interaction, rather than interaction with some narrative — novel versus game. Dating sims are simulation games and I wish people would stop confusing them with visual novels, they’re little alike.
Now, this brings us to Phoenix Wright and the question: is it an adventure game, or is it a visual novel?
Well, what does a (Japanese) adventure game look like? A world divided into screens, navigated by some sort of move command. On each screen you can perform a number of actions: look, talk, etc. Often, knowing when and where to use the items in your inventory is the core gameplay.
OH, OOOOOOOHH. The investigation part exactly matches the description of an adventure game. But wait, there are still the courtroom parts — those are much more text-heavy. Even there, the text only serves to frame the interrogation (gameplay) parts.
Does it even matter if Phoenix Wright is an adventure game or a visual novel? Why yes it does: using "visual novel" to refer to dialogue-heavy adventure/puzzle games quickly dilutes the term to meaninglessness. The whole point of calling things visual novels was to distinguish them from those types of games. I’m tired of seeing adventure games, simulation games and RPGs advertised as visual novels.
Phoenix Wright is at least similar enough to a VN that I won’t get omega-butthurt when it’s categorized as one. What I don’t like is when people drag it out as an example of what a visual novel is — because it isn’t.