Why do the writers of the anime ignore most post game content?

[skynerel]

In Advance Generation we got the battle frontier, but in Sinnoh it is completely avoided (Palmer did appear, but that's it), then in Unova there's no Black City battle tower or Pokemon World tournament (even though they created an entire extra arc to stall for time until the next saga), no battle maison in XY or Emma/Looker story (perhaps because the schedule was so tight), no battle tree or Rainbow Rocket in Alola and so on.

I guess since it's late content it has never been a priority, specially since by the time they would get to those places they need to start thinking of the following saga, but still it would've been nice to see The Battle Chatelaines, Rainbow Rocket or some other postgame story content adapted to the anime in some way.

 

[Hidden Mew]

I think it's mainly due to how by the time the anime finishes adapting the main storylines of each game, it's about time to wrap things up and move onto the next series. They tend to stretch out a whole journey over the course of a generation. Adapting the post storyline content could have been an interesting way to help extend a series, but I imagine that they're more focused on showcasing the main storyline content from the games moreso than the post game stuff.

In the case of DP, there simply wouldn't have been any time to properly do another Battle Frontier. They did it in AG basically because it was a way to promote both Emerald and FR/LG at the same time, but they didn't really need to do that for Platinum. More importantly, it just couldn't work since they had spent so much time on both Ash and Dawn's Gym and Contest journeys. I remember people saying that we could have gotten a Battle Frontier arc if there wasn't that year gap between Ash's seventh and eighth Gym badges, but I never understood that. Not only because there wasn't a huge amount of filler in that year, but a year for Ash to have multiple Frontier Brain battles and Dawn to have at least five Contests and a Grand Festival on top of it would have been way too much. It would have resulted in both Sinnoh and the Battle Frontier arcs feeling more rushed. DP benefits a lot from just being focused on one journey instead of multiple.

With BW, they did rush through a lot of the main storyline, but I don't think that having something like the Battle Tower or the Pokemon World tournament would have really worked, at least with giving them that much time. After the Unova League, I suspect that they wouldn't want to jump right back into another tournament arc. The battle areas are fun for video games and they could work in the anime, but in terms of buying them more time before the next generation, I suspect that they wouldn't help much. They might be more of an issue with the production coming right after battle heavy episodes too. The XY post storyline with Looker and Emma could have been cool, but with how different Team Flare was handled in the anime, I don't think that they would have had the time or perhaps were event interested in going in that direction. After the Team Flare arc, the last few episodes were devoted to wrapping things up for the cast and sending them off.

Team Rainbow Rocket would have been cool to see in the anime in a way, but I'm also not sure how well it could have worked. Even with the anime's own alternate universe episodes in mind, Ash and his friends facing off against alternate universe versions of evil team leaders who won in their worlds would be kind of hard to pull off. That kind of fun video game fanservice can work for a video game moreso than in a series like SM.