Aside from the reasons I mentioned earlier, I think selling the character to PC was likely done to either receive that upfront bonus or net a higher cut of profits. Additionally the model updates were likely funded by PC and there would be a greater incentive for them to push for those if they owned the character outright. In the mind of the invader, the deal could have likely been something like this
>keep the character, you have to pay for model updates, and you get less money (either from a lower cut than the other girls or no bonus for selling the character)
>sign character to PC, PC will handle the costs/organization of model updates, and you get more money (either a bonus for selling or a non-cucked cut of profits)
In the mind of someone young/naive, the whole "I'll make less money and will still have to shell out the big cash for new models/outfits" was not attractive and they didn't think they'd ever get terminated.