It can't.
Is this a statement of fact or faith?
In a completely anoxic environment in which temperatures remain nearly constant, there is no scientific reason I am aware of that soft tissue could not be preserved.
It would be a mistake to think that what has been left behind is anything like barely-decomposed animal tissue. This isn't like pulling a strip of preserved muscle from a dinosaur bone...
These are tiny separated "islands" of organic residue that were sealed so perfectly from necrotic agents that "something" other than rock or mineral was left over after all of these years.
It would be surprising to find intact DNA in these samples, but if they got that extraordinarily lucky, we'll know a lot more about dinosaurs in 10 years than we have learned in the previous 100.
Why not?
Oh really. Please share your insights on this absolute.
Please study some basic biology and/or organic chemistry before making ridiculous assertions such as this. Soft tissue decays as a result of bacterial action. It can be degraded as a result of reaction with oxygen in the atmosphere. Isolate a sample of soft tissue in an environment with limited oxygen and free from bacteria, and it can be preserved indefinitely. The inside of a bone could, under the correct fossilization conditions, be just such an environment.