They are different types of infectious agents. It is not too hard to kill HIV in a sample, but destroying infectious agents of prion diseases, like CJD, are much harder.
Here's one of the first articles that detailed the process of making GH from cadavers:
LIFE - Google Books
Note that this was before a purification step was added (whatever that was - I don't know) to get rid of the risks of prions). You're not going to be able to toss a couple pituitaries in your food processor with bacteriostatic water and inject it.
