What is *it* that Anisa does for him that some regular slut didn’t? He had pussy being thrown at him. It’s not like Anisa is some 10/10 anything special. He could have just took a much more beautiful girl and plucked her out of his DMs.
In my humble opinion, Anisa was just the one to present herself to him in real life, and I think she just brute-forced her way in.
Ian was just an awkward incel back then - despite having many options, I just don't think he'd be very good at "sealing the deal" with any self-respecting woman. It takes some basic effort to woo someone, and even though he had fame and money on his side, I don't really think he would ever put in the effort to try and actually develop a relationship with any girl past the "hey, what's up" messaging phase. I think he'd be extremely awkward and self-conscious during dates, and I think that he wouldn't actually have the balls to set up a first date and actually take the girl somewhere. I don't think he'd be capable to give any sort of attention to a woman, he'd just be too awkward or simply not give a shit about it. Also, let's not forget that, by one of her exes and by Anisa's own admission, Ian is a cheap fuck. Going out on a date to Dennys and making the girl pay for her shit didn't award him any points.
Anisa, on the other hand, was dead set on getting his money and clout. They met IRL at that faggy convention, and, even though she was dating someone else at the time, she basically did all the effort for him. She wanted to be with him. She was the one that did all the flirting. She was the one that set up "fun" dates. She wanted to give him her full attention. She wanted to suck his dick so he would be hers. And Ian, just as spineless then as he is now, just let it happen. And, since he never had any healthy relationships, he probably thought that was the way it was supposed to happen. And also, probably because he was just as arrogant and full of himself as he is now, he probably thought that he wasn't meant to do any effort and girls should just throw themselves at him - after all, he IS IDUBBBZ, THE CONTENT COP.
I'm sure Ian, during his prime, had at least a few options. But I think his awkwardness, his arrogance, his cheapness and his lack of willingness to make it even past the talking through DM's stage drove any decent woman away. But Anisa is no decent, self-respecting woman. She didn't want Ian for who he is, she wanted his money and clout. That's why their relationship is so fucking strange and everyone with half a braincell can see it.
Tl,DR: Anisa just wanted his clout and money, therefore she brute-forced her way into sucking his dick. Now that she's in control, she doesn't need to do it anymore. It's like the e-whore version of getting pregnant to lock down a man. She really is a succubus.