XFS not available for / partition on RHEL6, why?

Have a project in our development environment that requires XFS, so we installed RHEL7.

This project may be approved for production use soon, however RHEL7 doesn't have a working DoD STIG, so we'll have to use RHEL6.

Experimenting with RHEL6 with XFS in development environment and noticed I can set partitions like, /var or /opt to XFS, however not / . I googled around to see if I could find the reason why, and couldn't.

And at this time too, we can't justify Scalable File System Add-On Entitlement. We have to work with what we have.

thanks