Why so many bounty campaigns are extended is very simple but from my own point of view I can attach it to some factors which I list below.
unable to meet their softcap: some bounty campaign manager will extend their bounty campaign so as to create more awareness about the project in order to attract more investors to invest in the project so that they can meet the softcap. While some projects will meet the softcap but will be unable to get listed on the exchange due to one reason or the other. So by extending the bounty campaign they tend to meet up their setup goals.