The credential parameter on new-item is not actually supported for filesystems, so I'm not sure what you mean by "it works." It does NOT create the file as the passed user. In fact, the filesystem provider will say:
"The provider does not support the use of credentials. Perform the operation again without specifying credentials."
Taking an educated guess, I'd say you're trying to create a file with a different owner. PowerShell cannot do this on its own, so you'll need the following non-trivial script:
http://cosmoskey.blogspot.com/2010/07/setting-owner-on-acl-in-powershell.html
It works by enabling the SeBackup privilege for your security token (but you must already be an administrator.) This allows you to set any arbitrary owner on a file. Normally you can only change owner to administrators or your own account.
Oh, and this script is for powershell 2.0 only.