Because TV and radio are synchronous formats. The station only needs enough storage and bandwidth to play the one piece of media that they are playing right now

Because TV and radio are synchronous formats. The station only needs enough storage and bandwidth to play the one piece of media that they are playing right now, in this very moment. Once it's been broadcast once, it goes right in the trash unless they really need to record it for reruns or something. Online video requires you to be able to store every piece of video that has ever been uploaded, for the rest of eternity, and requires you to be able to play anything on-demand to ~600 million simultaneous connections, each requesting a different piece of media. You can't fund that with advertising. Google hasn't ever released figures, but the speculation consensus seems to be that youtube has never made a profit.