I doubt gen 7 is the least popular but it is still the current generation and the games are different. The games are far more story driven, which makes them a little more 'heavier'. Which takes a bit from the replay value since afterI completed one of the games, I need to give it a bit of rest to dive back into it again.
I also think the VC releases of R/B/Y and G/S/C and Pokémon Go may been a little overwhelming since those former games are known, and have a big nostalgia and Pokémon Go also fed into the original and 2nd gen of Pokémon.
Also don't forget that the lifespan of the 3DS is at its end, a lot of Pokémon fans are also nintendo fans, I see online atleast and since the Switch is out with loads of amazing games on a complete different console. It might look like gen 7 is a bit overlooked, especially with the lack of backwards compatibility with the (3)DS consoles and the Wii (U)
I think it's just a combination of a lot of things happening all at the same time. I think in a few years people might look back better on it, when it has sunk in and people have time to dive into it when things aren't so overcrowded. I personally also have trouble with completing and focusing on everything, since there is too much out there.