I am not sure that there were a lot of people that left bounties, we can't even assume that those remaining could be bots or such. Maybe the "many" you're referring to could be the investors since ICOs isn't doing that pretty great (at least a bit of good) these days/year. These could lead to a lot of failure of ICOs even the legit ones since investors could be too strict in what basket do they like their money to be thrown at. I still see a lot of bounties being participated by thousands of participants.