A big part of the reason is because B2/W2 are better as continuations of B/W.
Memory Link adds a lot of fanservice and connection to the previous games, such as;
>Being able to catch all of N's high-IV Pokémon
>Battling Cheren and Bianca with their updated teams
>References to the first player character by name
>Multiple cutscenes expanding the characters and world of the first games.
Then you consider what happened to the Region. B2/W2 still has the best regional Dex in the franchise, with them adding about 150~ fan-favorite Pokémon lines to Unova. You could now re-explore Unova in a far less linear fashion.
The characters were fantastic, as you got to see all your favorites return, wizened and more experienced. The new characters, Roxie and Colress in particular, are IMO some of the best in the entire franchise.
The biggest thing however, was the Post-Game. B2/W2 had without a doubt, the largest Post-Game in the entire franchise.
>White Tree Hollow/Black Tower Dungeons with high-level Pokémon Trainers to grind against & defeat, with a Shiny as a reward
>Tons of post-game dungeons and areas to explore
>N's Pokémon
>Pokémon World Tournament on top of the Battle Subway, fanservice to Gens 1-5, makes the Alola Battle Tree look like dogshit.
>PokéStar Studios is probably the best Pokémon sidegame we ever had, actually providing tons of in-game rewards.
>Catching them all earns you the Shiny Charm, making Shiny Hunting an actual valid Post-Game hobby for the first time
>Catching them all also earns you the chance to unlock a new area to catch a Shiny Haxorus as a reward.
And lastly the story, which is compelling mostly as a continuation to B/W.
So yes, B2/W2 are better games, once you take into account that you should really be playing them as an extension of the Unova experience.
