>>35565262That’s because it didn’t have a lot of stuff it NEEDED to add, just quality of life stuff and a new “story” focusing on the legendary beasts, specifically suicune, and their origins. USUM on the other hand had much more it needed to do to remedy SM’s perceived “lack of content” and while it dipped its toe in a ton of ideas that would’ve helped with its longevity it doesn’t actually really expand upon any of them. RR is a good start to a lengthier post game but it really just begins and ends there, the expanded upon ultra space is great on paper but in execution it’s just a warp to a brand new hallway with neither looker or anabel to deliver interesting story beats and delve into what makes a UB a UB. Fleshing out Necrozma and it’s motives was the only thing it really truly delivered on.
Not that it really matters, with HGSS around the only reason you’re playing crystal in 2018 is for nostalgia, early Phanpy or the VC Celebi.
Tl;Dr Crystal doesn’t have much new content because Gold and Silver didn’t need much else, USUM is viewed as lacking in comparison despite having “more new content” slimply because it needed more than it provided.