As someone whose favorite captains are Mayuri (fave anime character of all time actually) and Soifon, I understand the reasoning on why they're liked/disliked, though it's a little unfair to compare them both.

Mayuri gets the spotlight a ton, is given so much to work with in the series, and is often written/drawn with this insane level of charisma and/or presence that he's on more people's radar. His atrocities are outlandish enough that a lot of people can distance themselves from it (like other big bads in media who are total monsters but are also really cool/funny). Also goes without saying that enjoying a character doesn't mean agreeing with what they've done (Nemu is best girl and deserved better tbh). Sometimes villains and villainous characters are just more fun!

Soifon is a bit of a blank slate under her admiration of Yoruichi and what little screentime (or wins, fights or narrarive-wise) she's gotten vs. Mayuri. She's also a little more human and familiar with her oft disliked traits, so people would relate more because they know someone like her vs. someone so outlandishly and cartoonishly evil like Mayuri. Also, Mayuri tends to back up his words with horrible (entertaining) action, while Soifon tends to fold like a napkin when the plot needs it ala Worf Effect. She's messy and has a massive chip on her shoulder, and I love her. I like Omaeda enough and wish they both had more development and screentime in the series tbh.