I originally switched mains from shiek to pikachu upon finding out about tiers. In almost all games I like playing somewhat underpowered characters/classes for multiple reasons.
1) Their potential seems less explored. I like playing around with a character and figuring out my own style to them from the start. I'll take a few tactics here and there from other players, but it feels nice to feel like I've developed a lot of my style myself. As well, I like to push games to their limits and see all that is possible. Since high and top tier are more commonly played, you're bound to see all that they're capable because enough people will be trying to push the character to its limits. Low and bottom tiers are underrepresented, and so it's likely their potential won't be as well explored. I'm not out to prove that pikachu can be up their with higher characters, I think his tier placement is about right, but I'd like to be able to show what he's capable of. Sometimes I'll lose and come out of the match with a smile on my face because I'd have pulled off and interesting 0-death one stock that turned a few heads. Showing what he can do is just as important to me as showing how good he is as a character.
2) The challenge. It's a challenge to win, regardless of your character, but I like to up the difficulty a little bit. This ties a little bit into notoriety, my next point, but I get excited when I know i'm at a disadvantage from the start, and it makes winning with that starting disadvantage so much more meaningful to me. I'm aware that my character has a plateau far below that of other characters in terms of potential, but I don't necessarily have to be in the finals place very high as long as I feel I progressed farther than people exprected me to..
3) Notoriety. Yeah, the attention is nice, and it is somewhat easier to become known as a lowtier player than a high tier one, as long as you can get used to losing a lot. I'm honestly pretty bad at this game, but I've played pikachu enough to be comfortable with him and can pull off some pretty **** combos if I get the chance. Then people oooo and ahhh because they haven't seen pikachus do that before and they think I'm good because of it. So yeah, it's easier to become popular with a low tier character, and I'll admit I like the attention.
4) Playstyle. Pikachu is fun, what can I say. I didn't decide "I'm going to play the character that's at the 18th spot in the tiers" or whatever. I just knew I wanted a high-low tier char that was fun to play. He's fast, versatile, and has a lot of little interesting things to him. Back when I was picking him up, not a lot of people knew how the uair worked, and now it's common knowledge and a very important part of his gameplay, so figuring all that out and getting used to it was fun. I play young link for similar reasons, I like his playstyle, it feels natural.
5) Inexperience advantage. For a while I assumed this could somewhat cover for my character's inadequacies. Everyone knows how to play against foxes, sheiks, marths, etc. but few know what to do against a pikachu. It's less present nowadays since the game has been out for so long and most decent players have a general idea of how to deal with all chars, but there can be a decent advantage in your opponent not knowing what to expect. Since most of my matches would be against higher tiered characters, I'd likely have more experience in the matchup than them and would know what to watch out for and what I could do in a lot of situations, where as they would have to figure out a lot of this stuff during the match.
Nowadays, I mostly play him in tournaments because I'll do best with him. Yeah I've played a lot with top tier, in friendlies I've played more fox and marth than any other chars lately. But in tournaments, I have the most competative experience and invariably do better with pikachu than about any other character I could choose.