I could only speculate here but likely there is no specific reason for this particular API to not be exposed; just some functionality not implemented yet / API not covered on 100%. This applies for all plug-ins that are not automatically generated from some model - you usually start with only the most important/used API, and gradually add the remaining functionality during the next few plug-in releases.
The plug-in is developed not by vRO team but by some NSX team, and I'm not sure if they are monitoring this forum on a regular basis, so you may want to ask the same question also on NSX community forum at https://communities.vmware.com/community/vmtn/nsx