The problem is the old "slippery slope" analogy.
Once you allow that one guy int he NFL to use HGH (or whatever) the NFL has set a precedent and it will be hell for everyone in terms of trying to argue that "your guy" needs it, when maybe he doesn't, etc.
There would be roughly 90% of the NFL players claiming they need it to recover from various injuries.
Maybe they could only allow it in extreme cases where a guy had been I.R.'d and it's a blown ACL or other SEVERE ligament injuries. It'd have to be verified by maybe three separate, NFL-approved/sanctioned physicians who all three concur that the player DID receive a severe ligament injury, or whatever they deem is severe...and it would have to be administered by the NFL's docs at THEIR chosen facility and also witnessed by separate, impartial witnesses. Then they'd need to say "He can only take it for x-amount of time," then they test him after the treatment was to have ended to make sure he cleans it out of his system 100%. Then RE-TEST that player throughout the year to make sure he hasn't reverted back to taking it again.
Sounds like a damn difficult process, IMO, to make it available AND properly sanctioned by the NFL. Otherwise, every guy on every team is going to flood the NFL offices with petitions to use this "therapy" for what ails them.
Just my thoughts on why it could or could not be allowed in today's NFL.