It's all about expected disk sizes in 10 years time (when RHEL 7 main support ends). Ext4 scales up to around 16 TB -- I know it theoretically can support a much larger volume size, but in practice it doesn't work so well. XFS handles tens of terabytes without a problem and Red Hat has been supporting huge XFS volumes for years with many customers (who in earlier versions of RHEL paid extra for that support).