Phoenix Wright does something a lot of VNs don't: it gives the player the illusion that they're doing more than just paging through dialog and picking the right answer at prompt every once in a while. In other words, it masquerades as a more full-featured adventure game. 999 does a similar trick. Is it kind of stupid? Sure. Does it actually make for a significantly different experience from the Choose Your Own Adventure feel of most VNs? Not really, but it feels that way, and that's a key difference when it comes to getting people to sit down and pay attention, and take it seriously.
The appeal of something like Umineko, where the player is encouraged to speculate and try to solve the mysteries of the story on their own, is much closer to that of a "traditional" novel or other linear medium. That's not to say it isn't an enjoyable experience, horrific translation aside, just a different one from what is commonly expected of "videogames," and a harder sell to publishers and gamers.