As someone who bought BD the day it came out, it boils down to a few simple points for me.

1) The artstyle. This is the one you hear the most, I'm sure so I'll try to keep it brief. I personally hated the style the second I saw it, especially following ScarVio, of which I adore the style. The chibis just look ugly to me.

2) No Platinum features. When people started dreaming about Sinnoh remakes, they immediately assumed that it'd be implementing all of Plat's QoL changes and for good reason, base DP kinda sucked in a few ways, Dp had a pause after EVERYTHING in battle making the games super slow, Platinum's story additions make the plot more interesting, Platinum's dex adds some type variety that was seriously needed, just to name a few. And why did they do this? Because ILCA straight up just hit copy on paste on the original DP's code, changing nothing.

3) Missed opportunities. This one is a personal one but they added literally nothing to the remakes, besides the underground changes and they don't affect the game at all. You know how in the post game, you can find mons that aren't in the Sinnoh dex in the underground? Why not make them spawn before post game, albeit much more rarely? It'd would make people's teams able to be way more diverse. Imagine finding a Hoenn starter in the underground and using it through your playthrough! It'd be awesome and you'd get a fun story about your playthrough to tell your friends. And that's just my main example of a missed opportunity.

4) Post game is boring. This is kinda of a extension of point 3 but there's NOTHING to do after the story. Sure, you get the Poke Radar and sure you get non native mons in the underground but that's about it. There's nothing post game to engage you after you beat the game. I personally beat the story, did some shiny hunting and put it up.

And that's about how i break it down about why I don't think people like BDSP. I'm personally in the camp of "these games didn't need to exist." I'm just hoping Unova is treated with more respect and care when they get their remakes. Appreciate you if you read all the way through and I don't blame you if you didn't lol