That's because no one wants to touch the question of who's the designer. If it's God, this is just Creationism. If it's not God, then who?
If ID is going to go anywhere as a serious body of knowledge, it has to get past this hurdle. Then it has to define "complexity" as something more than "I know it when I see it". And it needs to provide more evidence than "biologists don't know something".
Is evolution still secular? It has entered into something it intentionally omits.
Unlesss they're theologians, why should the "who" make any difference? It would appear beyond the scope of science to ask that question.