Detective Pikachu's co-writers said the Detective Pikachu theme was decided on before they got involved

It's very clear that Pokemon Co. wanted to turn Detective Pikachu into a movie right from the start. In an interview with Comicbook.com, co-writer Benji Samit said Detective Pikachu as a theme was picked before they even were involved.

"When we first got involved, it was already Detective Pikachu. I think some of those conversations happened before us. Detective Pikachu is what Legendary and the Pokemon Company were in agreement of. If we're going to make a live action Pokemon movie, it needs to be different. It needs a reason to exist. We don't want to just remake movies we've already seen before.

I think they rightfully thought this weird side-game that most people were not familiar with was actually a really cool thing to see in live action. Ryme City and Pokemon and people living together is such a cinematic thing [and] that's what they wanted to do from very early on. When we first heard about it, we were like, 'Yeah, that sounds awesome!'"