Well maybe the reason why polkadot didnt conduct bounty campaign before is because they are confident that the project will still become successful even without the help of bounty hunters that will promote thier product.
It's the main reason. Just take a look at the majority of the big projects that have already backed by so many major companies and venture capital and they have gotten a lot of money to do the various promotion to attract the buyers or ico participants in the past. This is not only happening with the polkadot but a project that has gotten very big hype didn't even need to do promotion but otherwise, the ico hunters will always hunt this kind of project to make instant ROI from there. I have experienced this so many times since the ico was booming.