@Havoc Read what she said. Back then, she wanted to be rid of it all, but for some reason the Cluster creature was concerned, or at least cautious.
@Creator I think Mandark loved DeeDee. Loved her to the point, where he knew, DeeDee would never approve of such a thing. His parent are hippies, some of their beliefs may have rubbed off on him. He may have viewed such an act, equal to disrespecting her remains and her wishes.
I'm pretty sure deedee wouldn't have wanted Mandark and dexter to keep fighting after she died or even for Mandark to blame dexter for her death. But he did, and I'm sure that's something she wouldn't approve of. As for the organic material needed, I'm pretty sure in one of the dexters lab episodes Mandark had built a shrine to deedee complete with her hairbrush.
Also Mandark hated his parents, a lot. Why then of all times would their beliefs rub off on him?
Cuz DeeDee died. He believed in an angel for some time. Also, he blamed Dexter for her death. Not to mention he was mentally distraught, to the point where he committed suicide. His logic was twisted to better cope with himself. If he ever stared to build a DeeDee android, he'd probably feel all the guilt rush at him, which his minds was refusing.