One of the hosts on the Celtics reddit podcast put it really well:
Kyrie is a great, great basketball player. He's extremely talented.
He's not an honorable person. You wouldn't want your friends, your family, or your co-workers to carry themselves or act like Kyrie does. Hopefully, you wouldn't want to embody the kind of persona that Kyrie puts out for public consumption.
I think the same applies to KD.
They are both really great basketball players. But there's something lacking in who they are as people that makes it difficult to want to see them succeed.
I know that the counterargument would be something like -- we all loved Pierce, right? Well, Pierce is not exactly a saint. Same for Kevin Garnett. He's a trash talking maniac. But there's something about those guys, which I can't describe any other way than "charisma," that makes you want to follow them, to have them on your side.
I think Kyrie and KD both lack that. I also think there's a certain sullen, sulky pride to both Kyrie and KD, like they know they lack that charisma but they resent the fact that fans don't love them more, because they think their prowess on the court should be enough. But it isn't, and it won't ever be.