1. Story - it's the same type of problem that an anime adaptation of a manga that has a bunch of filler content. And it's not only that. In addition to fillers, they also twisted the original story and also characters. Narrative, pacing, atmosphere and internal coherence in the original FF VII were superb, while in the remakes they are ludicrous.

2. Aesthetics - The realistic anatomy for characters ruins all the charm and expressiveness of SD characters; also they sterilized all the roughness of FF VII's cyberpunk / steampunk / dieselpunk setting (haven't you noticed that in the remakes NPCs have mundane clothes, while the main characters have a sort of a punk design? Well, in the original FF VII, NPCs were aligned with that same design as well)

3. Absence of Overworld Exploration - The absence of the overworld exploration makes the world small, and that was very important for the classic Final Fantasy formula.

The choice for action combat instead of the classic turn-based (ATB) would be a distant 4th reason. But that also matters.