i'm guessing range was a big factor...100 yards accurately with air vs 200+ accurately with powder...
and pumping it back up in combat would not be a good thing to get caught doing when the enemy does a bayonet charge...
muskets were horribly inaccurate down to 100 yards anyways. The black smoke blocked the view after the first volley so aiming was out of the question..not to mention recoil.
The musket also needs to be propped up to reload..and there is no reason the air rifle could not be fitted with a bayonet (heck it needs no loading from the end of the barrel so it'd be even better to fix a bayonet on it).
The guy said the rifle would fire about 45 rounds before starting to lose PSI. It took 1500 pumps to get its pressure to full. Armies of the day would line up to fight each other...plenty of time in the night or morning to pump up your gun. Even then, most musket troops barely fired 10 to 20 rounds before either being dead or in bayonet combat.
The key thing is rate of fire. A line of foot firing 10 rounds a minute will obliterate a line firing avg 2 rounds a minute.
I think the reason why this weapon was not adopted was the same for which the fergusson rifle (or rifles in general) were not adopted: bureaucratic red tape and unwillingness in the army to spend money to re-arm their troops.
