Why is Luigi's Mansion not a good stage for Pikachu? I mean, I spam Dsmash in that stage.
Well, dsmash is never spammable against a good player, because they can easily DI out of it. I guess on LM they might bounce back into it off the ceiling (I can't remember how low the ceiling is on the first floor... I almost never play on this stage), but I doubt it. Either way, dsmash will never kill someone on this stage if they're on the bottom floor, and it probably won't rack up very much damage once it's stale.
Luigi's Mansion is bad because when the house is there, someone can flee to the bottom floor and pretty much avoid being killed. You can't thunder through the platforms, and usmash, fsmash, dsmash, and n-air will all bounce them off of the ceiling and into the floor if they're near the middle of the stage. I did some LM testing for my Killachu thread, and Mario didn't die from a fresh fsmash 'til something like 245% from the middle of the stage. For heavies like Bowser, it was closer to the low 300s.
Also, the second floor isn't very safe if your opponent has a long-range move that can hit you from below. Not only is it not safe, but you have to maneuver to the middle section or the sides to drop down, and movement in general is very hindered on this stage. For a character like Pika, who thrives on his speed and ease of moving around his opponent, this hurts his game a lot.
I'm sure there's a lot more reasons, but these are the most basic ones, I think.