Well, I also have both atm. Altrough I need to admit my DS Lite is only used as GBA console and for stuff that requires the GBA slot because of weird accesouries (like Guitar Hero On Tour).
I think it’s because of that. I play the old DS games on my new 3DS. And while the games did improve, the games on 3DS still wheren’t that advanced even for most of the time it was alive, since it laster quite long. So it easily feels more “backwards” than "last gen”. I also don’t see as much difference between them as the jump from PS1 to PS2 to PS3. Or the jump from GameBoy to DS serries, and 3DS to Switch for that matter. For the most part, the different DS’ feel more like different models than different consoles.
While the 3DS was released in 2010, the DS is only 6 years younger releasing in 2004. The hardware isn’t thát far apart. And while the last game for the 3DS was released in 2021, that still was made for at that moment 11 year old hardware (and by now 13 year old). And while the size of games may have quadruppeld between the first DS and the last, 4GB games where nothing in 2021. They bassically kept making games with restrictions of old hardware longer, rather than having a huge improvement.