because they can discuss things more candidly. While the public hearings only real purpose is to give the politicians something they can use on the campaign trial to show their constituents that they are "fighting for them on Capitol Hill." For various reasons politicians have come to the conclusion that if they don't get a certain amount of airtime on the C-SPAN channels, then their constituents are likely to conclude that means they are not fighting for them on Capitol Hill. Hence why you see so much grandstanding on any of the C-SPAN broadcasts.