Lack of hype is probably a big part of it, but that's a bit difficult to adjust considering Pika's playstyle. Since his moves are quick, light, and overall modest looking, there's less visual conveyance to the audience of where the hype is. The speed of the moves is of course fantastic for any fighting game, but because they are low in power and overall not very flashy it's hard to look like the kills are deserved. Lack of hitstun means limited combos, lack of range means limited control, and lack of startup and power on finishers means its harder to appreciate Pika's traditional fighting game KO methods to the casual observer. When a general audience gets hype over Pikachu (outside of QA which is movement so besides the point), it's usually not about the super quick, hard to miss Upsmash but the call of the flashy Thunder that comes after it. Tailspikes and gimps in general are nowhere near as powerful looking despite being more reliable and integral to Pika's play, and it's harder to appreciate the skill needed when just looking at it and seeing a weak hit net the same effect (KO) as a power move or a long, hard earned combo.
But the biggest reason for lack of players is simply because he's a hard character to play. Axe and Anther are extremely precise in their movement, spacing, and option selection, but even watching their play it seems to always feel like and uphill battle against most match-ups. Both players still make mistakes (Anther's particularly infamous for his SD's) and have frequently used secondaries. Outside of Pika's speed and recovery, there isn't much about him that conveys intimidation or conventional tactics. Small wavedash, negative priority on DD, small shield and grab, poor air mobility, low weight, lack of power for hitstun/clanks and lack of general range and disjoint forces immediate reliance on more technical things like SHFFL and QA and as little of screwing said tech up to make up for the lack of perceived power. Couple that with Pika's lack of hype and you get a character that's hard to pick up, hard to keep up, and hard to get the impression of getting reward out of it.
It's debatable how big of a flaw this is or not (considering Pika's still somewhat successful at the highest levels of play), but the fact is that Pika's just not a noob friendly character, and judging by the lack of Pika players in any game save maybe 64, that's just how he's always been. He's still a very solid character with balanced strengths and weaknesses, but when the rest of the cast has more room for error then you can see why he's not particularly used. People I play also describe him as hard to hit or living forever, so maybe from the opponent's view he may not be fun or fair to play against either. And when the classic casual impression of a Pika player is to spam Thunder it doesn't help the case very much.
I really wish for more Pika players at all, then maybe we can get more high level players from that. Until then just be flashy in your own ways. Use F-smash and Dair and even footstools more. Do crazy QA mixups. Go for the reckless Skull bash finish offstage! Push Pika in all directions, discovery as much "PMness" as we can, and maybe then other players will want to pick him up too.