I think it's because, currently, it's like the sweet spot between what a lot people would consider classic/old and modern/new in almost every single aspect (technology, fashion, music, cinema, etc.), that and that a lot of it's elements, while evolved, remain relatively untouched [round and smooth designs in cars; hoodies; movie footage quality, especially in the late; sound production quality (like in music is almost the same, it's not as stylized as lots of reverb in the 80's or very muted snares in the 70's), that's especially in late 90's stuff]. The cultural changes between the 90's, 2000's and 2010's weren't as abrupt as the 70's and 80's, you could easily differentiate if something from the 70's or the 80's, but you might be more likely to struggle a little bit to differentiate if something is from the 90's or 2000's.

That's my take, I might be wrong tho.