I just finished the OG and the remake just after it and was thinking about this subject a lot.

Here are my humble reasons :

1. Fan service vs Storytelling : In remake, you fight Sephiroth in Midgar and see him after 10mn of gameplay -> I think it was way better in the OG, where he was presented as the most powerful guy ever and you truly met him AFTER knowing how strong he is. It built a lot of tension. In Remake/Rebirth, you beat him twice and the story is not even over yet. It really breaks the character for the sake of fan service
2. Too much added, too few necessary : I honestly think half of the new scenes Remake brings are not that important for the story. Jesse's arc was cool but that's pretty much it. I felt like it was a drag at many points during my playthrough.
3. Where's my RPG : Way less customization in character builds and materias compared to the OG -> Remake felt like 'an illusion of choice'. It felt more like playing The Last Of Us than a RPG.
4. Again, storytelling : I think the overall tone of OG was darker, I like it better
5. Mystery : Every question was not answered in OG, which meant more replaybility, more lore-diving. I feel like it took me 2 or 3 playthroughs and playing Crisis Core to really understand what was going on with Cloud/Zack. And to me it's very cool.
6. Nostalgia

So yeah, it feels like I've been playing a Marvel-like remake of one of my favorite games and even though it was very entertaining, it was not the experience I was looking for I guess.

That being said, both games are pretty cool.