For all of its faults, Pokemon: Detective Pikachu really made audiences feel like they were in a Pokemon world and that was its greatest strength. This probably had a lot to do with Legendary working closely with The Pokemon Company, who made sure certain things remain faithful to the source material. Turns out that also came with limitations as VFX supervisor Pete Dionne told io9 a shocking limitation the company gave.
To make sure that the film was in the same universe as the games or anime, The Pokemon Company told Dionne that Pokemon should only have three or four attack moves. While there weren't a lot of Pokemon battles, there was one in the climax that (spoilers) had Pikachu take on Mewtwo.
Dionne revealed this in an interview with io9. Here's what he had to say:
Pokemon are only allowed to learn four moves in the games and The Pokemon Company probably told them to have three attacks so that they can have a support/buff skill. We didn't really see this in action but the few moves we saw were awesome, like Pikachu's Volt Tackle.
Hopefully, future Pokemon movies have better battles.
Pokemon: Detective Pikachu might still be showing in theaters.
Read:New Version of Detective Pikachu is Coming to Nintendo Switch, Will it Have Ryan Reynolds?