Using dbms_scheduler.create_credential on a exedata

Question

I want to setup a file_watcher for file created in a location in the OS. We are currently using Jobs that access the files through utl_file.fopen to access the files using a directory entry. I ask my DBA to setup a credential so I can establish the file watch using dbsm_scheduler. He told me that because we are using a exadata appliance for our database we could not create the credential. I dont get it! If we are accessing the file through a directory why is a issue access through a credential? Any Ideas?