Pikachu is rare in that he's a more difficult version of sheik- nice, quick, strong character with a heavy emphasis on neutral and damage combos.
His advantages are
Crazy damage output from combos (grab at 0 leads to ~60-70% if you react to DI correctly)
Quick Attack. Quick Attack is the best move in the game, by far. Better than witch time. It gets out of combos/disadvantage for free, starts combos at all %, recovers for free, and can hit them from across the stage quicker than people can react to. The only counterplay is for them to read when you use the attack, and if you'll hit them or where you'll land (which can be any point on stage, more or less), and throw out a lingering hitbox there.
Has multiple kill confirms
Best ledgeguarding in the game
All of pika's kill confirms are technical/tough to do consistently. This makes pika have one of the highest competitive skill floors for consistent tournament play.
* Pika's light and dies earlier, but this usually isn't an issue. About same weight as sheik.
I think there are 2 reasons there are so few pika players.
1) "Just play Sheik" Honestly, pika has a discount sheik stereotype, and it's not wrong in ways. Sheik doesn't have pika's kill confirms, QA, or smaller hurtbox, but has a relatively free neutral. And Pika's neutral is, in many eyes, his best advantage, so just go with the best neutral character in the game. (Personally, I think pika's other attributes make up for it, but I get this viewpoint.)
2) Pika is a technical character. I've had to play him for quite a few months before I got results, just because landing a kill under pressure is really difficult. Kill confirms are character specific, % specific, and rage specific, so going in to a matchup you have to know which ones will work and which won't.
I think Pika is potentially the toughest character in Smash, but with best play, easily top 5.
Oh, also, Pika beats the current top 5 IMO. Pika bodies diddy and cloud, no doubt. 60/40 and 70/30 respectively. Sheik is 50/50 or 45/55ish, maybe better. Never played against many sheiks. Pika Bayo is either 50/50 or 55/45. I'm leaning towards the latter. Pika combos bayo easier than bayo can combo pika (small hurtbox + SDI), better kill power too.
Pika's worst matchups are Mario and Ness. something like 40/60 each, maybe a bit worse. Upsmash (mario) and uair (ness) are safe, strong moves in neutral that can kill pika a bit early and also beat out Quick Attack.
In conclusion, there's a few things that are very broken (QA), but overall Pika's just a really strong character without any real weaknesses.