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.