I'd like to know why one cannot will disposition of ones body after death.
It is evident to me that what happens to your body after you die is not yours to decide. This is not right. How your body is disposed of by, say, cremation, is up to your next of kin or executor, who may decide to have you buried instead, or mounted in a college biology exhibit, for that matter, against your wishes.
No matter what you want, it's ultimately not your call but their call.
But even worse than that, your genes can be extracted and patented by people and/or corporations, and so your genetics are potentially up for sale in the developing age of clones and so forth.
On a theological level, this is condemnation to Hell, yes? This is pure real Satanism.