I can only speak to my personal preference and offer a theory that is tangentially related.
For me, I tend to favor Simon since he was in the original CastleVania, as well as CastleVania II: Simon's Quest and Super CastleVania IV back in the day. He has that classic appeal for me. Also, for whatever little it's worth, I just think Simon's design is cooler.
When it comes to Richter, in most regards I tend to favor Trevor from CastleVania III; I like his designs better, I like the different party members from CastleVania III with Sypha, Alucard, and Grant, and while it strays from the lore a lot I liked him in the Netflix show. Oh, and Trevor doesn't get brainwashed by Dracula's minions, which can't be said for Richter. I was actually kind of disappointed that Richter got the Echo Fighter spot in Super Smash Bros. instead of Trevor, though I realize Richter is more popular and probably makes more sense.
My theory: I would guess that there's a subset of people who like Simon for being the original, like myself. I would also guess that this might be influenced by age in some cases, people who grew up playing those earlier games and have the attachment toward him.
As far as Super Smash Bros. goes, as far as I'm aware, they're essentially the same in most regards, or at least started out that way. I haven't been paying particularly close attention to the patch notes, so I don't know if they've ended up playing any differently from one another, but I kind of doubt it. The only difference I found while looking around was in their down-Bs: Simon's down-B causes a fire and Richter's uses aura, meaning Simon's can set off Link's bombs with his and Richter's can't, while Richter's can damage Red Pikmin and Simon's can't. I can't say that seems like an important distinction, but hey, I'm not a tournament player, so I can't say with certainty that this doesn't influence any Simon favoritism in the community.
I'm not sure why whatever the popular opinion might or might not be should influence amiibo purchasing decisions though? If you like Richter better and are indifferent toward Simon, just stickter with Richter.
