Because it's not about "the cloud" in general. It's about specifically the rebroadcast of television radio signals, concerning which there's a substantial body of law created to govern the activities of cable TV providers.

In other words, the SC just defined Aereo as a cable company, when they wanted to be a cloud services company.

Thanks

..."And we have not considered whether the public performance right is in- fringed when the user of a service pays primarily for something other than the transmission of copyrighted works, such as the remote storage of content."