Air gun slugs vs black powder slugs

What's the difference in the two ?
i have noticed some very high end black powder slugs with ballistic tips and plastic gas rings the slugs are just slightly undersized to allow for easier loading in muzzle loaders. Thing is black powder slugs seem to be better made than what I've seen in air rifle slugs 
but looking around I don't see anyone really using them 
is their a reason for this?