Relaying on their own data centers and their own cloud stuff would have created a dependency between iCloud and the data center level stuff. If the data center project would have been delayed for a reason or another.. too bad for the iCloud.

Also it might not be a good idea to start testing your inhouse cloud infrastructure with a high profile product like iCloud that is likely to attract quite a few users in coming months.

I think it absolutely makes sense to do the development and initial launch with outside services and then later on maybe migrate to your own data centers and own cloud stack.