For example, CNN, ABC, etc. CNN has live streaming, but its only of press conferences, not their actual channel. I can't imagine their advertisers would care because they'd get more exposure and it would only benefit them if I can watch more Wolf Blitzer. The only people who might care are the cable TV companies.
I suspect it has to do with US law or cable company contracts because Al-Jazeera and NHK both have streams, although I know at least Al-Jazeera is broadcasted in the US.