Hypotheticaly speaking, lets say someone say Armstrong inject something into himself. How would they know what it was?
Yes you've proven he injected something into himself.
Next from a legal point of view you have to prove it was a banned substance, which is harder to do.
Remember they say it is better for ten guilty persons to go free than one innocent person to go to jail.
Supposedly USPS handed around white lunchboxes with doping products. If you're a USPS rider, you'll have a pretty good idea what's in Armstrong's package. Also if you see Armstrong transfusing half a litre of blood on the team bus, there's not much room for speculation.
Plus: You don't know what USADA knows. They might well have his suppliers as well.