You're overwriting your CSV each time, so you will only ever get the last result.
$esxcli.storage.core.device.list($RDMDisk.ScsiCanonicalName)| Select-Object Device, IsPerenniallyReserved | Export-Csv -NoTypeInformation -Path "C:\Users\cBurnett\SkyDrive\vSphere CLI Scripts\Set-RDMReservations\get-rdmreservations.csv"
Add the -Append switch and it should work I think.