The problem is that there is practically no reason to main Pikachu as long as Sheik exists. Pre-patch, these two--theoretically--had the same play style and character design. They ran circles in neutral and controlled it, racked up damage, but had difficulty finishing the job. In practice, Sheik had good KOing ability thanks to the 50/50. As such, Sheik's playstyle was notably different and superior in practice, as she barely had to commit to killing.
Post-patch, and the nerfs to Sheik have basically reduced her to an upgraded version of Pikachu--powerful neutral, versatile combo game, but sub-par KOing ability. Sheik still outclasses Pikachu in all these regards.
Furthermore, a perfect way to demonstrate all of Pikachu's flaws can be summed up in one scenario: in their last game at PAX South, Jband was at a 100% deficit, with ESAM at 13%. ESAM was one read away, or up throw, from taking the set. But then Jband landed a dash attack and carried him off the top. The reason Pikachu loses a number of top-tier MU's (
in particular, even after the nerfs) is mainly because his opponents can survive to unreal percentages, land a few hits, and KO Pikachu while sitting on a ton of rage.