Honestly, the Pokemon games are miles better than YW in everything but maybe aesthetics.
Pokémon games are boring, repetitive and run poorly. The first two generations were amazing because things were still fresh, now it's so fucking stale. I doubt you even played a YW game...
And lol at the "maybe" aesthetics part, YW games look a lot better, there's no doubt about that. The aliasing in Pokémon is some of the worst on the 3DS. The battle system is a complete borefest of spamming the same attack over and over again in SP. There's no strategy or tactics involved at all and the story is horrible.
Quality has little to do with sales in most cases.