Considering the set of Savin's *cough* «kinks» and the fact that he is a giant ass, even if he wasn't the main reason behind the fact that Einen didn’t maid it, he probably disliked her whole concept anyway.
Virgin monogamous companion? Fucking disgusting. Am I right, fellas?
You do realize that one of the mechanics behind Einin was to make her, with the help of Cait I may add, a giant slut that would choke on a minotaur dick with you. Which means, while yes, she could be your pure waifu, she could also be your slut in crime.
And considering what William said on /cocg/ (a general found on /vg/ board on 4chan), what he wrote that time was nothing related to her being a companion. He basically wrote her introduction scenes, as in, scene where we in process of recruiting her. She had no speaking options with us, about our companions or even a sex scene. And it is not like Savin asks much. Companions like Quin and Atugia are bare bones as hell (I think atugia has less content than many of the NPCs If I go just by observation alone) yet they are still there, in game. Former was even added even though Savin himself does not care for him and has asked Bubble multiply times to "fix" him by turning him into a girl. Bubbs obviously refused yet he is still in the game.
Like, Savin personally does not care for half of the stuff that is in this game yet he still allows it if the quality is good enough.
So lets not try to add makeshift fuel to non existing fire.
What I think happened was that William had a simple case of writing bloat. He just keeped adding and adding on the recruitment quest untill he simply got burned out by it all. And considering what he promised with Einin:
She could be your monogamous wife or poly wife.
You could enslave her.
All of that would be their own parsers.
She was supposed to be present in a lot of scenes involving other NPCs.
Now sure he could have gotten rid of half of that and still have a full companion. But Will does not strike me as a guy who would half ass his vision for sake of easing his workload. So in the ends, he just got burned out on working on her, and that is that.