Cause he’s a good villain. It‘s not really complex. He’s powerful, he’s played an important role, he’s got good lines, he’s had good scenes, his fighting style is cool. Sometimes simple is good. Akainu played a very simple role in Marineford and he excelled at it.
He’s also not one-dimensional, he showed some cunning with his Squard plan, he spared Kuzan, he’s shown in leading the Marines that he’s not just an attack dog, he’s shouted at the Gorosei, Sengoku’s laughing about the role not being as easy as it looks.
The good thing about Sakazuki is that Oda can just keep him the way he is, or he can expand on his past more. He’ll still be a great villain either way.