Because apparently getting 6 min of screentime in 4 episodes, along with an arc that happened to still feature every member of the team decently, means that it takes the spotlight away from them completely and overrides everything else in the region...
Literally Greninja is no different than any of Ash's other aces except that it's a mega and actually besides it's arc, it gets little spotlight. I'd say the league has focused more on the team, but Greninja being the ace and Pikachu being pikachu means they'll get the most time, even though it wasn't that much