Back when Gen 5 came out, not a lot of people liked it
It is a bit more complex than that i think.
You guys have to remember when BW first came out, you didn't really have access to pokémon from previous generations and the design of the new ones were heavily criticized.
Sure people aren't as passionate on hating their designs nowadays but they also don't have to use said pokémon nowadays either nor watch them be the focus. In Gen 5, that wasn't an option until Bank/BW2.
BW2 fixed a lot of the other issues people had with the generation by adding not only older pokémon, but also more optional routes and side content to make it less linear, more postgame, places to explore and a better facility, aswell as reducing the cutscenes/dialogue that stopped the flow of the gameplay, tho BW2 story wasn't as praised.
When it came to BW's story, the praise isn't really something recent, it was present even when it first came out, but since designs were revealed even before release, that is the first thing people remember reacting to.