1. It's not Microsoft's bread and butter. History has shown when it's no longer cost-effective, they'll drop it or neglect it in a heartbeat.
2. Say it does become wildly successful and pushes out its major competition (Dropbox & Drive), history has shown they'll still neglect it until it bit rots enough that a competitor can grab a foothold (eg. IE, Office, Hotmail, etc etc etc).
Microsoft has awesome, awesome engineering people. They just need to change their vision and their leadership.