The main reason behind bounty being preferable than mining is because of the hard work and money you need to invest on mining is considered the hard way to get that coin. Whereas, bounty programs offer free coins for completing the bounty tasks, even if you have buy reputable account to join, that won't cost you as much as mining.
Completely wrong, mining is not hard the only thing you need is a miner and cheap electricity and that is all, the reason people prefer bounties are the costs, mining has very high cost and you need to know what you are doing before you even begin to be profitable but getting bounties does not cost anything and it can be done in your free time that is why people prefer to do bounties instead.