I do trade for furniture a lot, so I burned myself out, tbf. Anyway, I feel like everyone else covered important reasons - the loss of individual character of iconic old series (the cute series just isn't the same thing as the lovely series, even though we have cute new colourways; I miss my blue chair and table; etc.), the feeling of incompleteness, missing favourite furniture items (I mourn the same stuff as everyone else, also pls give me back my sunflower in a vase and my old-style tea set), the excessive similarities in villager houses and even our own islands, etc.
But another reason I'm not super happy with the situation is because the game-as-service/patch model takes away one of the most comforting aspects of AC to me - it's always been a constant and rather safe. I might not have known everything that I could find, but I knew a lot of it, and that I would eventually get there, etc. New Horizons changed a lot for the better and I think it is the best game in the series, but now we have a lot of uncertainty over whether things will ever come back, or in what ways they'll have been changed...
On a personal level, I loved the aesthetic of New Leaf the most out of all the series entries. It had so many cute, unique looking, and pop cultureish items (partly due to all the promotions, like the 7/11 stuff, the Sanrio collab, and the regional events, but also just stuff like the hair bow wig, for example). To some extent that's still available in Pocket Camp, which lacks the social sim stuff I need out of AC. I do feel like a lot of that stuff is missing in AC:NH, but it's hard to go back to AC:NL. I'll never forget my excitement at seeing the mermaid furniture in the first real AC:NL trailer. I still sometimes boot up New Leaf just to look at my mermaid and pink rococo rooms.
We did get a lot of items back with the Nook's Cranny upgrade, so I'm hoping it'll continue to be the same, but they have to actually put them in the game, and with the global situation as it is right now, that's adding more uncertainty that's pretty rough. I do think the variations are often really clever in AC:NH though and I hope we'll get more diverse items with time. It's just hard to wait, and I don't know how long I'll play. Realistically I don't play a save file for a year nonstop.
I appreciate the opportunity to whine and to see everyone else's feelings. It's nice to let it all out.