The issue is that the three owners (Comcast, Disney, News Corp) provide the most popular content on the service. In order to become a truly independent platform, Hulu needs to beef up its offerings so that the loss of any individual content partner would not be a big deal.
They are not there yet, though they are moving in the right direction with Hulu Japan, the Univision deal, etc. As of now, however, the major content partners still hold a large amount of power and the potential buyers saw this from miles away.