Just a quick comparison, a simple backup tape can supposedly read and write data at 80mbps (some more, some less). That's about 10megabytes a second. Over the internet, your lucky to get 2megabytes a second.
So, even if we assumed a corporations upload was beefy enough to do backups to a cloud server in a single night. It would take 5 times longer to recover from a disaster using the cloud than just using backup tapes.