Almost certainly a matter of circumstances. Having 888 of them as part of the ritual, while deamons of Khorne are trying to get in seems rather like a recipe for disaster.Is there any particular reason why the Eldar wouldn't want to try and resurrect Khaine? Is it only the circumstances of Khaine's rebirth they object to, or is there another reason?
I'd have thought that Khaine might be able to tag team Slaanesh with Ynnead quite effectively, and would probably be able to distract Nurgle and enable Isha's rescue as well.