Probably has something to do with how unique every evolution tends to be. MC is pretty close to Ella but only early on when he's focusing on the Body aspects of it. As soon as he starts exploring the Memory aspect of his power he starts growing quite a bit differently than her. Ella hints that she's probably the only Superhuman that MIGHT be compatible with him that way but it's probably less to do with either one of them individually and more to do with how similar their Body power is. Ella's unique control over the inner workings because of her own experiments on herself and perks of being the Third's Chosen probably makes it more likely that she could control it to some degree, but MC just doesn't have that capability on his end (yet).Am I missing something?
Meanwhile Aglae can force it with the nature of her power, and I bet Eisheth could too if she were so inclined. Henri is also on the shortlist of beings that could probably do it if he wanted, but he DEFINITELY wouldn't want to. Truth/Fantasy/Chaos with enough power behind it can just get around the natural Order. I suspect Evolution can too but MC's not there yet.