Because doing bounty work is free and we could get coins for free and mining need investment for equipment and you need a good rig to make a good income. Like $3k worth of mining rig could just give you a profit of $300-$400/month not sure though but I think it would be near this range unlike bounty campaign $300-$400 is achievable or more if you land a good bounty ICO.
but currently following a bounty program can not be called free, why? because to have an account with a large enough ranking is not easy, some people even sell their account with a very expensive price, with uncertain income because a lot of fraud projects today.
True. It comes with a cost. First of all I wouldn't call it free because you are using electricity, spending time and effort whatever it is. That means you are paying that price in order for you to earn here in bounty.