There are actually a lot of theories, the leading one being that the narcissistic, self-serving son of a bitch got bored of Heidi. Additionally, he got technology back. For him, technology is like a relentlessly obedient servant, further tiring him with their relationship. As a result of his not caring, Heidi tells him she wants an “100-100 effort” relationship, which he obviously doesn’t want to commit to. So instead he goes fully back to being a narcissistic, insufferable, controlling, manipulative fat fuck. Sorry about the rudeness, I just wanted to rant about this a bit as well.

I believe that what Butters told Cartman at the end of Season 20 really affected their relationship in between Seasons 20 and 21. Cartman started to doubt Heidi, realizing that she isn't as funny as she is smart. I honestly hate Season 21. It's hard to watch Heidi being treated so badly until she breaks and becomes a worse Cartman. After Season 21, we have no idea what will happen with her or her relationships with her old friends. Hopefully she will have a greater presence in Season 24 or future seasons.

Idk but i hope Heidi is able to get back to normal and be happy again :3

@ItzyogirlLayla I hope so too. I think it's implied that she's doing better now, but we don't know for sure.

I agree with @IdkAUsernameIGuess , I honestly think Cartman just got sick of her. We learned he’s not very capable of handling a girlfriend, as when Heidi kept realizing how toxic their relationship was becoming and tried to break it off with him, he would cry and beg her to stay with him. Fuck him for that.

I don't know

Probably bcz he's an arsehole

Yeah, but you don’t have to randomly add that to a legitimate discussion about his relationship with Heidi. If you just dislike Cartman, just don’t say anything online in this legitimate discussion.

Oop wrong place to add it