N as a central character was a much more active than Lillie. As the King of Team Plasma, he was the driving force behind the story's conflict. By frequently challenging you to Battles, and aiming to take the Champion's throne before you, he also forges a rivalry with you. He is your antagonist, and his appearances are never a waste of time.
SM revolves around Lillie, but for most of the game she's just there. She doesn't do a-lot. Yet you're expected to like her, and care about her, and want to protect her. Her cutscenes don't really involve you, in the same way that N acted as an an obstacle and a force for you to overcome.
There's also a real lack of buildup and tension in SM.
WIth BW, you're constantly meeting with Plasma grunts on the road. You overhear their speech in Accumula, they ransack Lenora's Gym when looking for the Dragon Stone, they 'liberate' people's Pokemon, challenge you to Battles, etc.
You're prompted to work with the local Gym Leaders to take action in almost every major city, and you learn a little more about Plasma and N while doing so.
It all comes to ahead during that ever so iconic scene at N's Castle. N has become Champion before you, summons his goons, and the whole League rallies at your back. The culmination of an adventure.
In SM, nothing of consequence really happens until Ulaula Island. It's just this rosy little island adventure until Lillie goes to confront her mother. And Lusamine...Is not your villain. Aether Paradise has nothing to do with you. Defeating her isn't this climactic victory over a longtime enemy. You haven't even gotten under her skin, like we did with Giovanni, Maxie/Archie or Cyrus, for foiling their plans all over the map. You just walk in on some family bullshit and suddenly you have to fix their problems. That's why it feels like a 'movie'. Because unlike every other Pokemon game, you have nothing to do with the conflict at hand. This is someone else's story, and you're a glorified tourist.
