That's because people did the research and recorded the numbers. Litterally only three moves are different. Now there is actual evidence, rather then just subjective opinion.
The Electroshock Arm sends foes at a 45 degree angle while the Upperdash launches them strait up. Electroshock also does, uh, I think it was 0.5% more damage. Upperdash also kills at about 140%, while Electroshock's KO% depends on stage positioning. Upperdash kills more consistently, though, and is a good set up for juggles. This is why Pit's side special is considered superior.
Electroshock can OHKO Luma, however, but it leaves you open.
The Silver Bow has far less curving ability, but does 2% damage extra damage at full charge and, uh, I think it was 0.25% more damage uncharged. It also has just a tiny bit more knockback. The extra damage is hardly noticeable, but the curvature is. Again, Pit's arrows are considered superior.
The side tilt is an add one. They both look the same, have the same sweet spot, and do the same amount of damage. But for whatever reason, a sweet spotted side tilt from Pit can kill at a decent percentage. Dark Pit's can't. I have no idea what that's about, but Pit's is undeniably better.
...I got too into this.