I think you might have some bias going that makes you perceive this dramatic difference in quality? The only thing that comes to my mind:
Borislav Slavov (the composer) recently said that he made instrumentation choices in BG3 that sort of reflect the BG1+2 soundtracks (heavy brass + choir, harp motif in the beginning etc.), but I find that there's enough "exotic" instruments, including synths + effects still in BG3 so you can clearly hear it's not just "generic orchestra".
For me the quality of BG3's compositions are just as top notch as DOS2, and in BG3 especially the transitions between different pieces as you do things in the game are even better than in DOS.
More is to come of course - we haven't heard much yet.
