Most animals are extinct because us as humans think it is fine to kill them for there tusks or skin. Some species of animals go extinct because they die off from a natural cause. To answer your question animal cruelty is mostly the reason they are going extinct.
You would have to be very careful of cross-contamination. That means you would have to work very carefully and precisely with protective clothing, ensuring that none of your DNA (from skin or hair or even trace) gets into the sample you are working with
You would have to be very careful to check and record samples properly to ensure nothing got mixed up or mislabeled and this could be disastrous for criminal cases
All the material is very precious and sometimes only one sample exists, so you have to avoid making any mistakes. Experiments can't always be repeated.
