My best guess: whatever they're filing now was so exhaustively researched that it took months to prepare the strongest case they're able to make, possibly delayed by the lawyers working on several other cases. Plus waiting until sales have dried up can maximize damages.

Another possibility is that Nintendo/TPC is planning to make some big Pokémon announcements soon and wants to target this shortly before their own new games to reduce competition. Palworld might seem like more of a threat to the execs now that Pokémon is nearing a major release than it was in the middle of a long drought for the series.