Because Microsoft want to push us all to the cloud and have great marketing speakers, however for partners who use it in practice it's far from ideal. I don't even recommend much cloud solutions to clients. Group policy in a AD domain. Anyone...? 
What do you think of the issue that the sync relation just stops working and does not sync anymore, almost experienced it at every OneDrive client, and the client itself won't notice anything although you are telling that green checks have to be there. In case of a normal on-premise server: it works or it doesn't. If somebody can't put data on their file share they can not even see it in most cases.
I'm already waiting soooooo long on the ability to Sync shared folders and/or team sites. And I also have clients who only want to use OneDrive (to keep it simple) and no Sharepoint....