Because, as they are, those cells would not grow into an individual, even if they were kept alive.
...there is substantial evidence that discordance between MZ twins in fact has a genetic component. MZ twins develop from a separation of the embryonic cells at any point from the two-cell stage up until as late as day 8 when the primordial streak has already started to form.[4] The timing of the separation has consequences for the genetic composition of each twin, as the cells become more heterogeneous with time with respect to the mitochondria they carry [5] and the pattern of methylation, and hence potential gene expression, they display.[6]I wonder if Archbishop Carnley would be willing to change his deadline a little. Maybe, if he is looking for some arbitrary cutoff date, he should say that embryos up to 8 days, not 14 days, can be destroyed.
OK, good answer. So let's flesh this scenario out further. I know we're getting into hypotheticals now, but please indulge me nonetheless: what happens if researchers figure out how to perfect cloning to the extent that any stem cell (such as can be isolated from bone marrow or cord blood) can be induced to develop into a zygote? Does that change our medical ethics for bone marrow biopsy? Because then, of course, any of those cells has the potential to become an individual if kept alive.