Paying either in Bitcoin or ethereum is very costly for the project owners. They might in short of the funds at beginning and the project might not even raise the given amount required for bounty. If they pay via tokens, users who wants to buy these will set the value so literally the project owners won't be losing any money.
yeah because developer must allocated Ethereum or BTC for hunter and take some proceeds from sales to allocate and pay for hunters.
it's efficient for developer and that's why usually developer paying hunter using token